associations.rb

require 'active_support/core_ext/enumerable'
require 'active_support/core_ext/string/conversions'
require 'active_support/core_ext/module/remove_method'
require 'active_record/errors'

module ActiveRecord
  class AssociationNotFoundError < ConfigurationError #:nodoc:
    def initialize(record = nil, association_name = nil)
      if record && association_name
        super("Association named '#{association_name}' was not found on #{record.class.name}; perhaps you misspelled it?")
      else
        super("Association was not found.")
      end
    end
  end

  class InverseOfAssociationNotFoundError < ActiveRecordError #:nodoc:
    def initialize(reflection = nil, associated_class = nil)
      if reflection
        super("Could not find the inverse association for #{reflection.name} (#{reflection.options[:inverse_of].inspect} in #{associated_class.nil? ? reflection.class_name : associated_class.name})")
      else
        super("Could not find the inverse association.")
      end
    end
  end

  class HasManyThroughAssociationNotFoundError < ActiveRecordError #:nodoc:
    def initialize(owner_class_name = nil, reflection = nil)
      if owner_class_name && reflection
        super("Could not find the association #{reflection.options[:through].inspect} in model #{owner_class_name}")
      else
        super("Could not find the association.")
      end
    end
  end

  class HasManyThroughAssociationPolymorphicSourceError < ActiveRecordError #:nodoc:
    def initialize(owner_class_name = nil, reflection = nil, source_reflection = nil)
      if owner_class_name && reflection && source_reflection
        super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' on the polymorphic object '#{source_reflection.class_name}##{source_reflection.name}' without 'source_type'. Try adding 'source_type: \"#{reflection.name.to_s.classify}\"' to 'has_many :through' definition.")
      else
        super("Cannot have a has_many :through association.")
      end
    end
  end

  class HasManyThroughAssociationPolymorphicThroughError < ActiveRecordError #:nodoc:
    def initialize(owner_class_name = nil, reflection = nil)
      if owner_class_name && reflection
        super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' which goes through the polymorphic association '#{owner_class_name}##{reflection.through_reflection.name}'.")
      else
        super("Cannot have a has_many :through association.")
      end
    end
  end

  class HasManyThroughAssociationPointlessSourceTypeError < ActiveRecordError #:nodoc:
    def initialize(owner_class_name = nil, reflection = nil, source_reflection = nil)
      if owner_class_name && reflection && source_reflection
        super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' with a :source_type option if the '#{reflection.through_reflection.class_name}##{source_reflection.name}' is not polymorphic. Try removing :source_type on your association.")
      else
        super("Cannot have a has_many :through association.")
      end
    end
  end

  class HasOneThroughCantAssociateThroughCollection < ActiveRecordError #:nodoc:
    def initialize(owner_class_name = nil, reflection = nil, through_reflection = nil)
      if owner_class_name && reflection && through_reflection
        super("Cannot have a has_one :through association '#{owner_class_name}##{reflection.name}' where the :through association '#{owner_class_name}##{through_reflection.name}' is a collection. Specify a has_one or belongs_to association in the :through option instead.")
      else
        super("Cannot have a has_one :through association.")
      end
    end
  end

  class HasOneAssociationPolymorphicThroughError < ActiveRecordError #:nodoc:
    def initialize(owner_class_name = nil, reflection = nil)
      if owner_class_name && reflection
        super("Cannot have a has_one :through association '#{owner_class_name}##{reflection.name}' which goes through the polymorphic association '#{owner_class_name}##{reflection.through_reflection.name}'.")
      else
        super("Cannot have a has_one :through association.")
      end
    end
  end

  class HasManyThroughSourceAssociationNotFoundError < ActiveRecordError #:nodoc:
    def initialize(reflection = nil)
      if reflection
        through_reflection      = reflection.through_reflection
        source_reflection_names = reflection.source_reflection_names
        source_associations     = reflection.through_reflection.klass._reflections.keys
        super("Could not find the source association(s) #{source_reflection_names.collect(&:inspect).to_sentence(:two_words_connector => ' or ', :last_word_connector => ', or ', :locale => :en)} in model #{through_reflection.klass}. Try 'has_many #{reflection.name.inspect}, :through => #{through_reflection.name.inspect}, :source => <name>'. Is it one of #{source_associations.to_sentence(:two_words_connector => ' or ', :last_word_connector => ', or ', :locale => :en)}?")
      else
        super("Could not find the source association(s).")
      end
    end
  end

  class ThroughCantAssociateThroughHasOneOrManyReflection < ActiveRecordError #:nodoc:
    def initialize(owner = nil, reflection = nil)
      if owner && reflection
        super("Cannot modify association '#{owner.class.name}##{reflection.name}' because the source reflection class '#{reflection.source_reflection.class_name}' is associated to '#{reflection.through_reflection.class_name}' via :#{reflection.source_reflection.macro}.")
      else
        super("Cannot modify association.")
      end
    end
  end

  class HasManyThroughCantAssociateThroughHasOneOrManyReflection < ThroughCantAssociateThroughHasOneOrManyReflection #:nodoc:
  end

  class HasOneThroughCantAssociateThroughHasOneOrManyReflection < ThroughCantAssociateThroughHasOneOrManyReflection #:nodoc:
  end

  class HasManyThroughCantAssociateNewRecords < ActiveRecordError #:nodoc:
    def initialize(owner = nil, reflection = nil)
      if owner && reflection
        super("Cannot associate new records through '#{owner.class.name}##{reflection.name}' on '#{reflection.source_reflection.class_name rescue nil}##{reflection.source_reflection.name rescue nil}'. Both records must have an id in order to create the has_many :through record associating them.")
      else
        super("Cannot associate new records.")
      end
    end
  end

  class HasManyThroughCantDissociateNewRecords < ActiveRecordError #:nodoc:
    def initialize(owner = nil, reflection = nil)
      if owner && reflection
        super("Cannot dissociate new records through '#{owner.class.name}##{reflection.name}' on '#{reflection.source_reflection.class_name rescue nil}##{reflection.source_reflection.name rescue nil}'. Both records must have an id in order to delete the has_many :through record associating them.")
      else
        super("Cannot dissociate new records.")
      end
    end
  end

  class ThroughNestedAssociationsAreReadonly < ActiveRecordError #:nodoc:
    def initialize(owner = nil, reflection = nil)
      if owner && reflection
        super("Cannot modify association '#{owner.class.name}##{reflection.name}' because it goes through more than one other association.")
      else
        super("Through nested associations are read-only.")
      end
    end
  end

  class HasManyThroughNestedAssociationsAreReadonly < ThroughNestedAssociationsAreReadonly #:nodoc:
  end

  class HasOneThroughNestedAssociationsAreReadonly < ThroughNestedAssociationsAreReadonly #:nodoc:
  end

  class EagerLoadPolymorphicError < ActiveRecordError #:nodoc:
    def initialize(reflection = nil)
      if reflection
        super("Cannot eagerly load the polymorphic association #{reflection.name.inspect}")
      else
        super("Eager load polymorphic error.")
      end
    end
  end

  class ReadOnlyAssociation < ActiveRecordError #:nodoc:
    def initialize(reflection = nil)
      if reflection
        super("Cannot add to a has_many :through association. Try adding to #{reflection.through_reflection.name.inspect}.")
      else
        super("Read-only reflection error.")
      end
    end
  end

  # This error is raised when trying to destroy a parent instance in N:1 or 1:1 associations
  # (has_many, has_one) when there is at least 1 child associated instance.
  # ex: if @project.tasks.size > 0, DeleteRestrictionError will be raised when trying to destroy @project
  class DeleteRestrictionError < ActiveRecordError #:nodoc:
    def initialize(name = nil)
      if name
        super("Cannot delete record because of dependent #{name}")
      else
        super("Delete restriction error.")
      end
    end
  end

  # See ActiveRecord::Associations::ClassMethods for documentation.
  module Associations # :nodoc:
    extend ActiveSupport::Autoload
    extend ActiveSupport::Concern

    # These classes will be loaded when associations are created.
    # So there is no need to eager load them.
    autoload :Association
    autoload :SingularAssociation
    autoload :CollectionAssociation
    autoload :ForeignAssociation
    autoload :CollectionProxy

    autoload :BelongsToAssociation
    autoload :BelongsToPolymorphicAssociation
    autoload :HasManyAssociation
    autoload :HasManyThroughAssociation
    autoload :HasOneAssociation
    autoload :HasOneThroughAssociation
    autoload :ThroughAssociation

    module Builder #:nodoc:
      autoload :Association,           'active_record/associations/builder/association'
      autoload :SingularAssociation,   'active_record/associations/builder/singular_association'
      autoload :CollectionAssociation, 'active_record/associations/builder/collection_association'

      autoload :BelongsTo,           'active_record/associations/builder/belongs_to'
      autoload :HasOne,              'active_record/associations/builder/has_one'
      autoload :HasMany,             'active_record/associations/builder/has_many'
      autoload :HasAndBelongsToMany, 'active_record/associations/builder/has_and_belongs_to_many'
    end

    eager_autoload do
      autoload :Preloader
      autoload :JoinDependency
      autoload :AssociationScope
      autoload :AliasTracker
    end

    # Returns the association instance for the given name, instantiating it if it doesn't already exist
    def association(name) #:nodoc:
      association = association_instance_get(name)

      if association.nil?
        unless reflection = self.class._reflect_on_association(name)
          raise AssociationNotFoundError.new(self, name)
        end
        association = reflection.association_class.new(self, reflection)
        association_instance_set(name, association)
      end

      association
    end

    def association_cached?(name) # :nodoc
      @association_cache.key?(name)
    end

    def initialize_dup(*) # :nodoc:
      @association_cache = {}
      super
    end

    def reload(*) # :nodoc:
      clear_association_cache
      super
    end

    private
      # Clears out the association cache.
      def clear_association_cache # :nodoc:
        @association_cache.clear if persisted?
      end

      def init_internals # :nodoc:
        @association_cache = {}
        super
      end

      # Returns the specified association instance if it exists, nil otherwise.
      def association_instance_get(name)
        @association_cache[name]
      end

      # Set the specified association instance.
      def association_instance_set(name, association)
        @association_cache[name] = association
      end

    # \Associations are a set of macro-like class methods for tying objects together through
    # foreign keys. They express relationships like "Project has one Project Manager"
    # or "Project belongs to a Portfolio". Each macro adds a number of methods to the
    # class which are specialized according to the collection or association symbol and the
    # options hash. It works much the same way as Ruby's own <tt>attr*</tt>
    # methods.
    #
    #   class Project < ActiveRecord::Base
    #     belongs_to              :portfolio
    #     has_one                 :project_manager
    #     has_many                :milestones
    #     has_and_belongs_to_many :categories
    #   end
    #
    # The project class now has the following methods (and more) to ease the traversal and
    # manipulation of its relationships:
    # * <tt>Project#portfolio, Project#portfolio=(portfolio), Project#portfolio.nil?</tt>
    # * <tt>Project#project_manager, Project#project_manager=(project_manager), Project#project_manager.nil?,</tt>
    # * <tt>Project#milestones.empty?, Project#milestones.size, Project#milestones, Project#milestones<<(milestone),</tt>
    #   <tt>Project#milestones.delete(milestone), Project#milestones.destroy(milestone), Project#milestones.find(milestone_id),</tt>
    #   <tt>Project#milestones.build, Project#milestones.create</tt>
    # * <tt>Project#categories.empty?, Project#categories.size, Project#categories, Project#categories<<(category1),</tt>
    #   <tt>Project#categories.delete(category1), Project#categories.destroy(category1)</tt>
    #
    # === A word of warning
    #
    # Don't create associations that have the same name as instance methods of
    # <tt>ActiveRecord::Base</tt>. Since the association adds a method with that name to
    # its model, it will override the inherited method and break things.
    # For instance, +attributes+ and +connection+ would be bad choices for association names.
    #
    # == Auto-generated methods
    # See also Instance Public methods below for more details.
    #
    # === Singular associations (one-to-one)
    #                                     |            |  belongs_to  |
    #   generated methods                 | belongs_to | :polymorphic | has_one
    #   ----------------------------------+------------+--------------+---------
    #   other(force_reload=false)         |     X      |      X       |    X
    #   other=(other)                     |     X      |      X       |    X
    #   build_other(attributes={})        |     X      |              |    X
    #   create_other(attributes={})       |     X      |              |    X
    #   create_other!(attributes={})      |     X      |              |    X
    #
    # ===Collection associations (one-to-many / many-to-many)
    #                                     |       |          | has_many
    #   generated methods                 | habtm | has_many | :through
    #   ----------------------------------+-------+----------+----------
    #   others(force_reload=false)        |   X   |    X     |    X
    #   others=(other,other,...)          |   X   |    X     |    X
    #   other_ids                         |   X   |    X     |    X
    #   other_ids=(id,id,...)             |   X   |    X     |    X
    #   others<<                          |   X   |    X     |    X
    #   others.push                       |   X   |    X     |    X
    #   others.concat                     |   X   |    X     |    X
    #   others.build(attributes={})       |   X   |    X     |    X
    #   others.create(attributes={})      |   X   |    X     |    X
    #   others.create!(attributes={})     |   X   |    X     |    X
    #   others.size                       |   X   |    X     |    X
    #   others.length                     |   X   |    X     |    X
    #   others.count                      |   X   |    X     |    X
    #   others.sum(*args)                 |   X   |    X     |    X
    #   others.empty?                     |   X   |    X     |    X
    #   others.clear                      |   X   |    X     |    X
    #   others.delete(other,other,...)    |   X   |    X     |    X
    #   others.delete_all                 |   X   |    X     |    X
    #   others.destroy(other,other,...)   |   X   |    X     |    X
    #   others.destroy_all                |   X   |    X     |    X
    #   others.find(*args)                |   X   |    X     |    X
    #   others.exists?                    |   X   |    X     |    X
    #   others.distinct                   |   X   |    X     |    X
    #   others.reset                      |   X   |    X     |    X
    #
    # === Overriding generated methods
    #
    # Association methods are generated in a module that is included into the model class,
    # which allows you to easily override with your own methods and call the original
    # generated method with +super+. For example:
    #
    #   class Car < ActiveRecord::Base
    #     belongs_to :owner
    #     belongs_to :old_owner
    #     def owner=(new_owner)
    #       self.old_owner = self.owner
    #       super
    #     end
    #   end
    #
    # If your model class is <tt>Project</tt>, the module is
    # named <tt>Project::GeneratedAssociationMethods</tt>. The GeneratedAssociationMethods module is
    # included in the model class immediately after the (anonymous) generated attributes methods
    # module, meaning an association will override the methods for an attribute with the same name.
    #
    # == Cardinality and associations
    #
    # Active Record associations can be used to describe one-to-one, one-to-many and many-to-many
    # relationships between models. Each model uses an association to describe its role in
    # the relation. The +belongs_to+ association is always used in the model that has
    # the foreign key.
    #
    # === One-to-one
    #
    # Use +has_one+ in the base, and +belongs_to+ in the associated model.
    #
    #   class Employee < ActiveRecord::Base
    #     has_one :office
    #   end
    #   class Office < ActiveRecord::Base
    #     belongs_to :employee    # foreign key - employee_id
    #   end
    #
    # === One-to-many
    #
    # Use +has_many+ in the base, and +belongs_to+ in the associated model.
    #
    #   class Manager < ActiveRecord::Base
    #     has_many :employees
    #   end
    #   class Employee < ActiveRecord::Base
    #     belongs_to :manager     # foreign key - manager_id
    #   end
    #
    # === Many-to-many
    #
    # There are two ways to build a many-to-many relationship.
    #
    # The first way uses a +has_many+ association with the <tt>:through</tt> option and a join model, so
    # there are two stages of associations.
    #
    #   class Assignment < ActiveRecord::Base
    #     belongs_to :programmer  # foreign key - programmer_id
    #     belongs_to :project     # foreign key - project_id
    #   end
    #   class Programmer < ActiveRecord::Base
    #     has_many :assignments
    #     has_many :projects, through: :assignments
    #   end
    #   class Project < ActiveRecord::Base
    #     has_many :assignments
    #     has_many :programmers, through: :assignments
    #   end
    #
    # For the second way, use +has_and_belongs_to_many+ in both models. This requires a join table
    # that has no corresponding model or primary key.
    #
    #   class Programmer < ActiveRecord::Base
    #     has_and_belongs_to_many :projects       # foreign keys in the join table
    #   end
    #   class Project < ActiveRecord::Base
    #     has_and_belongs_to_many :programmers    # foreign keys in the join table
    #   end
    #
    # Choosing which way to build a many-to-many relationship is not always simple.
    # If you need to work with the relationship model as its own entity,
    # use <tt>has_many :through</tt>. Use +has_and_belongs_to_many+ when working with legacy schemas or when
    # you never work directly with the relationship itself.
    #
    # == Is it a +belongs_to+ or +has_one+ association?
    #
    # Both express a 1-1 relationship. The difference is mostly where to place the foreign
    # key, which goes on the table for the class declaring the +belongs_to+ relationship.
    #
    #   class User < ActiveRecord::Base
    #     # I reference an account.
    #     belongs_to :account
    #   end
    #
    #   class Account < ActiveRecord::Base
    #     # One user references me.
    #     has_one :user
    #   end
    #
    # The tables for these classes could look something like:
    #
    #   CREATE TABLE users (
    #     id int(11) NOT NULL auto_increment,
    #     account_id int(11) default NULL,
    #     name varchar default NULL,
    #     PRIMARY KEY  (id)
    #   )
    #
    #   CREATE TABLE accounts (
    #     id int(11) NOT NULL auto_increment,
    #     name varchar default NULL,
    #     PRIMARY KEY  (id)
    #   )
    #
    # == Unsaved objects and associations
    #
    # You can manipulate objects and associations before they are saved to the database, but
    # there is some special behavior you should be aware of, mostly involving the saving of
    # associated objects.
    #
    # You can set the <tt>:autosave</tt> option on a <tt>has_one</tt>, <tt>belongs_to</tt>,
    # <tt>has_many</tt>, or <tt>has_and_belongs_to_many</tt> association. Setting it
    # to +true+ will _always_ save the members, whereas setting it to +false+ will
    # _never_ save the members. More details about <tt>:autosave</tt> option is available at
    # AutosaveAssociation.
    #
    # === One-to-one associations
    #
    # * Assigning an object to a +has_one+ association automatically saves that object and
    #   the object being replaced (if there is one), in order to update their foreign
    #   keys - except if the parent object is unsaved (<tt>new_record? == true</tt>).
    # * If either of these saves fail (due to one of the objects being invalid), an
    #   <tt>ActiveRecord::RecordNotSaved</tt> exception is raised and the assignment is
    #   cancelled.
    # * If you wish to assign an object to a +has_one+ association without saving it,
    #   use the <tt>build_association</tt> method (documented below). The object being
    #   replaced will still be saved to update its foreign key.
    # * Assigning an object to a +belongs_to+ association does not save the object, since
    #   the foreign key field belongs on the parent. It does not save the parent either.
    #
    # === Collections
    #
    # * Adding an object to a collection (+has_many+ or +has_and_belongs_to_many+) automatically
    #   saves that object, except if the parent object (the owner of the collection) is not yet
    #   stored in the database.
    # * If saving any of the objects being added to a collection (via <tt>push</tt> or similar)
    #   fails, then <tt>push</tt> returns +false+.
    # * If saving fails while replacing the collection (via <tt>association=</tt>), an
    #   <tt>ActiveRecord::RecordNotSaved</tt> exception is raised and the assignment is
    #   cancelled.
    # * You can add an object to a collection without automatically saving it by using the
    #   <tt>collection.build</tt> method (documented below).
    # * All unsaved (<tt>new_record? == true</tt>) members of the collection are automatically
    #   saved when the parent is saved.
    #
    # == Customizing the query
    #
    # \Associations are built from <tt>Relation</tt>s, and you can use the <tt>Relation</tt> syntax
    # to customize them. For example, to add a condition:
    #
    #   class Blog < ActiveRecord::Base
    #     has_many :published_posts, -> { where published: true }, class_name: 'Post'
    #   end
    #
    # Inside the <tt>-> { ... }</tt> block you can use all of the usual <tt>Relation</tt> methods.
    #
    # === Accessing the owner object
    #
    # Sometimes it is useful to have access to the owner object when building the query. The owner
    # is passed as a parameter to the block. For example, the following association would find all
    # events that occur on the user's birthday:
    #
    #   class User < ActiveRecord::Base
    #     has_many :birthday_events, ->(user) { where starts_on: user.birthday }, class_name: 'Event'
    #   end
    #
    # Note: Joining, eager loading and preloading of these associations is not fully possible.
    # These operations happen before instance creation and the scope will be called with a +nil+ argument.
    # This can lead to unexpected behavior and is deprecated.
    #
    # == Association callbacks
    #
    # Similar to the normal callbacks that hook into the life cycle of an Active Record object,
    # you can also define callbacks that get triggered when you add an object to or remove an
    # object from an association collection.
    #
    #   class Project
    #     has_and_belongs_to_many :developers, after_add: :evaluate_velocity
    #
    #     def evaluate_velocity(developer)
    #       ...
    #     end
    #   end
    #
    # It's possible to stack callbacks by passing them as an array. Example:
    #
    #   class Project
    #     has_and_belongs_to_many :developers,
    #                             after_add: [:evaluate_velocity, Proc.new { |p, d| p.shipping_date = Time.now}]
    #   end
    #
    # Possible callbacks are: +before_add+, +after_add+, +before_remove+ and +after_remove+.
    #
    # If any of the +before_add+ callbacks throw an exception, the object will not be
    # added to the collection.
    #
    # Similarly, if any of the +before_remove+ callbacks throw an exception, the object
    # will not be removed from the collection.
    #
    # == Association extensions
    #
    # The proxy objects that control the access to associations can be extended through anonymous
    # modules. This is especially beneficial for adding new finders, creators, and other
    # factory-type methods that are only used as part of this association.
    #
    #   class Account < ActiveRecord::Base
    #     has_many :people do
    #       def find_or_create_by_name(name)
    #         first_name, last_name = name.split(" ", 2)
    #         find_or_create_by(first_name: first_name, last_name: last_name)
    #       end
    #     end
    #   end
    #
    #   person = Account.first.people.find_or_create_by_name("David Heinemeier Hansson")
    #   person.first_name # => "David"
    #   person.last_name  # => "Heinemeier Hansson"
    #
    # If you need to share the same extensions between many associations, you can use a named
    # extension module.
    #
    #   module FindOrCreateByNameExtension
    #     def find_or_create_by_name(name)
    #       first_name, last_name = name.split(" ", 2)
    #       find_or_create_by(first_name: first_name, last_name: last_name)
    #     end
    #   end
    #
    #   class Account < ActiveRecord::Base
    #     has_many :people, -> { extending FindOrCreateByNameExtension }
    #   end
    #
    #   class Company < ActiveRecord::Base
    #     has_many :people, -> { extending FindOrCreateByNameExtension }
    #   end
    #
    # Some extensions can only be made to work with knowledge of the association's internals.
    # Extensions can access relevant state using the following methods (where +items+ is the
    # name of the association):
    #
    # * <tt>record.association(:items).owner</tt> - Returns the object the association is part of.
    # * <tt>record.association(:items).reflection</tt> - Returns the reflection object that describes the association.
    # * <tt>record.association(:items).target</tt> - Returns the associated object for +belongs_to+ and +has_one+, or
    #   the collection of associated objects for +has_many+ and +has_and_belongs_to_many+.
    #
    # However, inside the actual extension code, you will not have access to the <tt>record</tt> as
    # above. In this case, you can access <tt>proxy_association</tt>. For example,
    # <tt>record.association(:items)</tt> and <tt>record.items.proxy_association</tt> will return
    # the same object, allowing you to make calls like <tt>proxy_association.owner</tt> inside
    # association extensions.
    #
    # == Association Join Models
    #
    # Has Many associations can be configured with the <tt>:through</tt> option to use an
    # explicit join model to retrieve the data. This operates similarly to a
    # +has_and_belongs_to_many+ association. The advantage is that you're able to add validations,
    # callbacks, and extra attributes on the join model. Consider the following schema:
    #
    #   class Author < ActiveRecord::Base
    #     has_many :authorships
    #     has_many :books, through: :authorships
    #   end
    #
    #   class Authorship < ActiveRecord::Base
    #     belongs_to :author
    #     belongs_to :book
    #   end
    #
    #   @author = Author.first
    #   @author.authorships.collect { |a| a.book } # selects all books that the author's authorships belong to
    #   @author.books                              # selects all books by using the Authorship join model
    #
    # You can also go through a +has_many+ association on the join model:
    #
    #   class Firm < ActiveRecord::Base
    #     has_many   :clients
    #     has_many   :invoices, through: :clients
    #   end
    #
    #   class Client < ActiveRecord::Base
    #     belongs_to :firm
    #     has_many   :invoices
    #   end
    #
    #   class Invoice < ActiveRecord::Base
    #     belongs_to :client
    #   end
    #
    #   @firm = Firm.first
    #   @firm.clients.flat_map { |c| c.invoices } # select all invoices for all clients of the firm
    #   @firm.invoices                            # selects all invoices by going through the Client join model
    #
    # Similarly you can go through a +has_one+ association on the join model:
    #
    #   class Group < ActiveRecord::Base
    #     has_many   :users
    #     has_many   :avatars, through: :users
    #   end
    #
    #   class User < ActiveRecord::Base
    #     belongs_to :group
    #     has_one    :avatar
    #   end
    #
    #   class Avatar < ActiveRecord::Base
    #     belongs_to :user
    #   end
    #
    #   @group = Group.first
    #   @group.users.collect { |u| u.avatar }.compact # select all avatars for all users in the group
    #   @group.avatars                                # selects all avatars by going through the User join model.
    #
    # An important caveat with going through +has_one+ or +has_many+ associations on the
    # join model is that these associations are *read-only*. For example, the following
    # would not work following the previous example:
    #
    #   @group.avatars << Avatar.new   # this would work if User belonged_to Avatar rather than the other way around
    #   @group.avatars.delete(@group.avatars.last)  # so would this
    #
    # == Setting Inverses
    #
    # If you are using a +belongs_to+ on the join model, it is a good idea to set the
    # <tt>:inverse_of</tt> option on the +belongs_to+, which will mean that the following example
    # works correctly (where <tt>tags</tt> is a +has_many+ <tt>:through</tt> association):
    #
    #   @post = Post.first
    #   @tag = @post.tags.build name: "ruby"
    #   @tag.save
    #
    # The last line ought to save the through record (a <tt>Tagging</tt>). This will only work if the
    # <tt>:inverse_of</tt> is set:
    #
    #   class Tagging < ActiveRecord::Base
    #     belongs_to :post
    #     belongs_to :tag, inverse_of: :taggings
    #   end
    #
    # If you do not set the <tt>:inverse_of</tt> record, the association will
    # do its best to match itself up with the correct inverse. Automatic
    # inverse detection only works on <tt>has_many</tt>, <tt>has_one</tt>, and
    # <tt>belongs_to</tt> associations.
    #
    # Extra options on the associations, as defined in the
    # <tt>AssociationReflection::INVALID_AUTOMATIC_INVERSE_OPTIONS</tt> constant, will
    # also prevent the association's inverse from being found automatically.
    #
    # The automatic guessing of the inverse association uses a heuristic based
    # on the name of the class, so it may not work for all associations,
    # especially the ones with non-standard names.
    #
    # You can turn off the automatic detection of inverse associations by setting
    # the <tt>:inverse_of</tt> option to <tt>false</tt> like so:
    #
    #   class Tagging < ActiveRecord::Base
    #     belongs_to :tag, inverse_of: false
    #   end
    #
    # == Nested \Associations
    #
    # You can actually specify *any* association with the <tt>:through</tt> option, including an
    # association which has a <tt>:through</tt> option itself. For example:
    #
    #   class Author < ActiveRecord::Base
    #     has_many :posts
    #     has_many :comments, through: :posts
    #     has_many :commenters, through: :comments
    #   end
    #
    #   class Post < ActiveRecord::Base
    #     has_many :comments
    #   end
    #
    #   class Comment < ActiveRecord::Base
    #     belongs_to :commenter
    #   end
    #
    #   @author = Author.first
    #   @author.commenters # => People who commented on posts written by the author
    #
    # An equivalent way of setting up this association this would be:
    #
    #   class Author < ActiveRecord::Base
    #     has_many :posts
    #     has_many :commenters, through: :posts
    #   end
    #
    #   class Post < ActiveRecord::Base
    #     has_many :comments
    #     has_many :commenters, through: :comments
    #   end
    #
    #   class Comment < ActiveRecord::Base
    #     belongs_to :commenter
    #   end
    #
    # When using a nested association, you will not be able to modify the association because there
    # is not enough information to know what modification to make. For example, if you tried to
    # add a <tt>Commenter</tt> in the example above, there would be no way to tell how to set up the
    # intermediate <tt>Post</tt> and <tt>Comment</tt> objects.
    #
    # == Polymorphic \Associations
    #
    # Polymorphic associations on models are not restricted on what types of models they
    # can be associated with. Rather, they specify an interface that a +has_many+ association
    # must adhere to.
    #
    #   class Asset < ActiveRecord::Base
    #     belongs_to :attachable, polymorphic: true
    #   end
    #
    #   class Post < ActiveRecord::Base
    #     has_many :assets, as: :attachable         # The :as option specifies the polymorphic interface to use.
    #   end
    #
    #   @asset.attachable = @post
    #
    # This works by using a type column in addition to a foreign key to specify the associated
    # record. In the Asset example, you'd need an +attachable_id+ integer column and an
    # +attachable_type+ string column.
    #
    # Using polymorphic associations in combination with single table inheritance (STI) is
    # a little tricky. In order for the associations to work as expected, ensure that you
    # store the base model for the STI models in the type column of the polymorphic
    # association. To continue with the asset example above, suppose there are guest posts
    # and member posts that use the posts table for STI. In this case, there must be a +type+
    # column in the posts table.
    #
    # Note: The <tt>attachable_type=</tt> method is being called when assigning an +attachable+.
    # The +class_name+ of the +attachable+ is passed as a String.
    #
    #   class Asset < ActiveRecord::Base
    #     belongs_to :attachable, polymorphic: true
    #
    #     def attachable_type=(class_name)
    #        super(class_name.constantize.base_class.to_s)
    #     end
    #   end
    #
    #   class Post < ActiveRecord::Base
    #     # because we store "Post" in attachable_type now dependent: :destroy will work
    #     has_many :assets, as: :attachable, dependent: :destroy
    #   end
    #
    #   class GuestPost < Post
    #   end
    #
    #   class MemberPost < Post
    #   end
    #
    # == Caching
    #
    # All of the methods are built on a simple caching principle that will keep the result
    # of the last query around unless specifically instructed not to. The cache is even
    # shared across methods to make it even cheaper to use the macro-added methods without
    # worrying too much about performance at the first go.
    #
    #   project.milestones             # fetches milestones from the database
    #   project.milestones.size        # uses the milestone cache
    #   project.milestones.empty?      # uses the milestone cache
    #   project.milestones(true).size  # fetches milestones from the database
    #   project.milestones             # uses the milestone cache
    #
    # == Eager loading of associations
    #
    # Eager loading is a way to find objects of a certain class and a number of named associations.
    # It is one of the easiest ways to prevent the dreaded N+1 problem in which fetching 100
    # posts that each need to display their author triggers 101 database queries. Through the
    # use of eager loading, the number of queries will be reduced from 101 to 2.
    #
    #   class Post < ActiveRecord::Base
    #     belongs_to :author
    #     has_many   :comments
    #   end
    #
    # Consider the following loop using the class above:
    #
    #   Post.all.each do |post|
    #     puts "Post:            " + post.title
    #     puts "Written by:      " + post.author.name
    #     puts "Last comment on: " + post.comments.first.created_on
    #   end
    #
    # To iterate over these one hundred posts, we'll generate 201 database queries. Let's
    # first just optimize it for retrieving the author:
    #
    #   Post.includes(:author).each do |post|
    #
    # This references the name of the +belongs_to+ association that also used the <tt>:author</tt>
    # symbol. After loading the posts, find will collect the +author_id+ from each one and load
    # all the referenced authors with one query. Doing so will cut down the number of queries
    # from 201 to 102.
    #
    # We can improve upon the situation further by referencing both associations in the finder with:
    #
    #   Post.includes(:author, :comments).each do |post|
    #
    # This will load all comments with a single query. This reduces the total number of queries
    # to 3. In general, the number of queries will be 1 plus the number of associations
    # named (except if some of the associations are polymorphic +belongs_to+ - see below).
    #
    # To include a deep hierarchy of associations, use a hash:
    #
    #   Post.includes(:author, { comments: { author: :gravatar } }).each do |post|
    #
    # The above code will load all the comments and all of their associated
    # authors and gravatars. You can mix and match any combination of symbols,
    # arrays, and hashes to retrieve the associations you want to load.
    #
    # All of this power shouldn't fool you into thinking that you can pull out huge amounts
    # of data with no performance penalty just because you've reduced the number of queries.
    # The database still needs to send all the data to Active Record and it still needs to
    # be processed. So it's no catch-all for performance problems, but it's a great way to
    # cut down on the number of queries in a situation as the one described above.
    #
    # Since only one table is loaded at a time, conditions or orders cannot reference tables
    # other than the main one. If this is the case, Active Record falls back to the previously
    # used LEFT OUTER JOIN based strategy. For example:
    #
    #   Post.includes([:author, :comments]).where(['comments.approved = ?', true])
    #
    # This will result in a single SQL query with joins along the lines of:
    # <tt>LEFT OUTER JOIN comments ON comments.post_id = posts.id</tt> and
    # <tt>LEFT OUTER JOIN authors ON authors.id = posts.author_id</tt>. Note that using conditions
    # like this can have unintended consequences.
    # In the above example posts with no approved comments are not returned at all, because
    # the conditions apply to the SQL statement as a whole and not just to the association.
    #
    # You must disambiguate column references for this fallback to happen, for example
    # <tt>order: "author.name DESC"</tt> will work but <tt>order: "name DESC"</tt> will not.
    #
    # If you want to load all posts (including posts with no approved comments) then write
    # your own LEFT OUTER JOIN query using ON
    #
    #   Post.joins("LEFT OUTER JOIN comments ON comments.post_id = posts.id AND comments.approved = '1'")
    #
    # In this case it is usually more natural to include an association which has conditions defined on it:
    #
    #   class Post < ActiveRecord::Base
    #     has_many :approved_comments, -> { where approved: true }, class_name: 'Comment'
    #   end
    #
    #   Post.includes(:approved_comments)
    #
    # This will load posts and eager load the +approved_comments+ association, which contains
    # only those comments that have been approved.
    #
    # If you eager load an association with a specified <tt>:limit</tt> option, it will be ignored,
    # returning all the associated objects:
    #
    #   class Picture < ActiveRecord::Base
    #     has_many :most_recent_comments, -> { order('id DESC').limit(10) }, class_name: 'Comment'
    #   end
    #
    #   Picture.includes(:most_recent_comments).first.most_recent_comments # => returns all associated comments.
    #
    # Eager loading is supported with polymorphic associations.
    #
    #   class Address < ActiveRecord::Base
    #     belongs_to :addressable, polymorphic: true
    #   end
    #
    # A call that tries to eager load the addressable model
    #
    #   Address.includes(:addressable)
    #
    # This will execute one query to load the addresses and load the addressables with one
    # query per addressable type.
    # For example if all the addressables are either of class Person or Company then a total
    # of 3 queries will be executed. The list of addressable types to load is determined on
    # the back of the addresses loaded. This is not supported if Active Record has to fallback
    # to the previous implementation of eager loading and will raise <tt>ActiveRecord::EagerLoadPolymorphicError</tt>.
    # The reason is that the parent model's type is a column value so its corresponding table
    # name cannot be put in the +FROM+/+JOIN+ clauses of that query.
    #
    # == Table Aliasing
    #
    # Active Record uses table aliasing in the case that a table is referenced multiple times
    # in a join. If a table is referenced only once, the standard table name is used. The
    # second time, the table is aliased as <tt>#{reflection_name}_#{parent_table_name}</tt>.
    # Indexes are appended for any more successive uses of the table name.
    #
    #   Post.joins(:comments)
    #   # => SELECT ... FROM posts INNER JOIN comments ON ...
    #   Post.joins(:special_comments) # STI
    #   # => SELECT ... FROM posts INNER JOIN comments ON ... AND comments.type = 'SpecialComment'
    #   Post.joins(:comments, :special_comments) # special_comments is the reflection name, posts is the parent table name
    #   # => SELECT ... FROM posts INNER JOIN comments ON ... INNER JOIN comments special_comments_posts
    #
    # Acts as tree example:
    #
    #   TreeMixin.joins(:children)
    #   # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
    #   TreeMixin.joins(children: :parent)
    #   # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
    #                               INNER JOIN parents_mixins ...
    #   TreeMixin.joins(children: {parent: :children})
    #   # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
    #                               INNER JOIN parents_mixins ...
    #                               INNER JOIN mixins childrens_mixins_2
    #
    # Has and Belongs to Many join tables use the same idea, but add a <tt>_join</tt> suffix:
    #
    #   Post.joins(:categories)
    #   # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
    #   Post.joins(categories: :posts)
    #   # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
    #                              INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories
    #   Post.joins(categories: {posts: :categories})
    #   # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
    #                              INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories
    #                              INNER JOIN categories_posts categories_posts_join INNER JOIN categories categories_posts_2
    #
    # If you wish to specify your own custom joins using <tt>joins</tt> method, those table
    # names will take precedence over the eager associations:
    #
    #   Post.joins(:comments).joins("inner join comments ...")
    #   # => SELECT ... FROM posts INNER JOIN comments_posts ON ... INNER JOIN comments ...
    #   Post.joins(:comments, :special_comments).joins("inner join comments ...")
    #   # => SELECT ... FROM posts INNER JOIN comments comments_posts ON ...
    #                              INNER JOIN comments special_comments_posts ...
    #                              INNER JOIN comments ...
    #
    # Table aliases are automatically truncated according to the maximum length of table identifiers
    # according to the specific database.
    #
    # == Modules
    #
    # By default, associations will look for objects within the current module scope. Consider:
    #
    #   module MyApplication
    #     module Business
    #       class Firm < ActiveRecord::Base
    #         has_many :clients
    #       end
    #
    #       class Client < ActiveRecord::Base; end
    #     end
    #   end
    #
    # When <tt>Firm#clients</tt> is called, it will in turn call
    # <tt>MyApplication::Business::Client.find_all_by_firm_id(firm.id)</tt>.
    # If you want to associate with a class in another module scope, this can be done by
    # specifying the complete class name.
    #
    #   module MyApplication
    #     module Business
    #       class Firm < ActiveRecord::Base; end
    #     end
    #
    #     module Billing
    #       class Account < ActiveRecord::Base
    #         belongs_to :firm, class_name: "MyApplication::Business::Firm"
    #       end
    #     end
    #   end
    #
    # == Bi-directional associations
    #
    # When you specify an association there is usually an association on the associated model
    # that specifies the same relationship in reverse. For example, with the following models:
    #
    #    class Dungeon < ActiveRecord::Base
    #      has_many :traps
    #      has_one :evil_wizard
    #    end
    #
    #    class Trap < ActiveRecord::Base
    #      belongs_to :dungeon
    #    end
    #
    #    class EvilWizard < ActiveRecord::Base
    #      belongs_to :dungeon
    #    end
    #
    # The +traps+ association on +Dungeon+ and the +dungeon+ association on +Trap+ are
    # the inverse of each other and the inverse of the +dungeon+ association on +EvilWizard+
    # is the +evil_wizard+ association on +Dungeon+ (and vice-versa). By default,
    # Active Record can guess the inverse of the association based on the name
    # of the class. The result is the following:
    #
    #    d = Dungeon.first
    #    t = d.traps.first
    #    d.object_id == t.dungeon.object_id # => true
    #
    # The +Dungeon+ instances +d+ and <tt>t.dungeon</tt> in the above example refer to
    # the same in-memory instance since the association matches the name of the class.
    # The result would be the same if we added +:inverse_of+ to our model definitions:
    #
    #    class Dungeon < ActiveRecord::Base
    #      has_many :traps, inverse_of: :dungeon
    #      has_one :evil_wizard, inverse_of: :dungeon
    #    end
    #
    #    class Trap < ActiveRecord::Base
    #      belongs_to :dungeon, inverse_of: :traps
    #    end
    #
    #    class EvilWizard < ActiveRecord::Base
    #      belongs_to :dungeon, inverse_of: :evil_wizard
    #    end
    #
    # There are limitations to <tt>:inverse_of</tt> support:
    #
    # * does not work with <tt>:through</tt> associations.
    # * does not work with <tt>:polymorphic</tt> associations.
    # * for +belongs_to+ associations +has_many+ inverse associations are ignored.
    #
    # For more information, see the documentation for the +:inverse_of+ option.
    #
    # == Deleting from associations
    #
    # === Dependent associations
    #
    # +has_many+, +has_one+ and +belongs_to+ associations support the <tt>:dependent</tt> option.
    # This allows you to specify that associated records should be deleted when the owner is
    # deleted.
    #
    # For example:
    #
    #     class Author
    #       has_many :posts, dependent: :destroy
    #     end
    #     Author.find(1).destroy # => Will destroy all of the author's posts, too
    #
    # The <tt>:dependent</tt> option can have different values which specify how the deletion
    # is done. For more information, see the documentation for this option on the different
    # specific association types. When no option is given, the behavior is to do nothing
    # with the associated records when destroying a record.
    #
    # Note that <tt>:dependent</tt> is implemented using Rails' callback
    # system, which works by processing callbacks in order. Therefore, other
    # callbacks declared either before or after the <tt>:dependent</tt> option
    # can affect what it does.
    #
    # Note that <tt>:dependent</tt> option is ignored for +has_one+ <tt>:through</tt> associations.
    #
    # === Delete or destroy?
    #
    # +has_many+ and +has_and_belongs_to_many+ associations have the methods <tt>destroy</tt>,
    # <tt>delete</tt>, <tt>destroy_all</tt> and <tt>delete_all</tt>.
    #
    # For +has_and_belongs_to_many+, <tt>delete</tt> and <tt>destroy</tt> are the same: they
    # cause the records in the join table to be removed.
    #
    # For +has_many+, <tt>destroy</tt> and <tt>destroy_all</tt> will always call the <tt>destroy</tt> method of the
    # record(s) being removed so that callbacks are run. However <tt>delete</tt> and <tt>delete_all</tt> will either
    # do the deletion according to the strategy specified by the <tt>:dependent</tt> option, or
    # if no <tt>:dependent</tt> option is given, then it will follow the default strategy.
    # The default strategy is to do nothing (leave the foreign keys with the parent ids set), except for
    # +has_many+ <tt>:through</tt>, where the default strategy is <tt>delete_all</tt> (delete
    # the join records, without running their callbacks).
    #
    # There is also a <tt>clear</tt> method which is the same as <tt>delete_all</tt>, except that
    # it returns the association rather than the records which have been deleted.
    #
    # === What gets deleted?
    #
    # There is a potential pitfall here: +has_and_belongs_to_many+ and +has_many+ <tt>:through</tt>
    # associations have records in join tables, as well as the associated records. So when we
    # call one of these deletion methods, what exactly should be deleted?
    #
    # The answer is that it is assumed that deletion on an association is about removing the
    # <i>link</i> between the owner and the associated object(s), rather than necessarily the
    # associated objects themselves. So with +has_and_belongs_to_many+ and +has_many+
    # <tt>:through</tt>, the join records will be deleted, but the associated records won't.
    #
    # This makes sense if you think about it: if you were to call <tt>post.tags.delete(Tag.find_by(name: 'food'))</tt>
    # you would want the 'food' tag to be unlinked from the post, rather than for the tag itself
    # to be removed from the database.
    #
    # However, there are examples where this strategy doesn't make sense. For example, suppose
    # a person has many projects, and each project has many tasks. If we deleted one of a person's
    # tasks, we would probably not want the project to be deleted. In this scenario, the delete method
    # won't actually work: it can only be used if the association on the join model is a
    # +belongs_to+. In other situations you are expected to perform operations directly on
    # either the associated records or the <tt>:through</tt> association.
    #
    # With a regular +has_many+ there is no distinction between the "associated records"
    # and the "link", so there is only one choice for what gets deleted.
    #
    # With +has_and_belongs_to_many+ and +has_many+ <tt>:through</tt>, if you want to delete the
    # associated records themselves, you can always do something along the lines of
    # <tt>person.tasks.each(&:destroy)</tt>.
    #
    # == Type safety with <tt>ActiveRecord::AssociationTypeMismatch</tt>
    #
    # If you attempt to assign an object to an association that doesn't match the inferred
    # or specified <tt>:class_name</tt>, you'll get an <tt>ActiveRecord::AssociationTypeMismatch</tt>.
    #
    # == Options
    #
    # All of the association macros can be specialized through options. This makes cases
    # more complex than the simple and guessable ones possible.
    module ClassMethods
      # Specifies a one-to-many association. The following methods for retrieval and query of
      # collections of associated objects will be added:
      #
      # +collection+ is a placeholder for the symbol passed as the +name+ argument, so
      # <tt>has_many :clients</tt> would add among others <tt>clients.empty?</tt>.
      #
      # [collection(force_reload = false)]
      #   Returns an array of all the associated objects.
      #   An empty array is returned if none are found.
      # [collection<<(object, ...)]
      #   Adds one or more objects to the collection by setting their foreign keys to the collection's primary key.
      #   Note that this operation instantly fires update SQL without waiting for the save or update call on the
      #   parent object, unless the parent object is a new record.
      # [collection.delete(object, ...)]
      #   Removes one or more objects from the collection by setting their foreign keys to +NULL+.
      #   Objects will be in addition destroyed if they're associated with <tt>dependent: :destroy</tt>,
      #   and deleted if they're associated with <tt>dependent: :delete_all</tt>.
      #
      #   If the <tt>:through</tt> option is used, then the join records are deleted (rather than
      #   nullified) by default, but you can specify <tt>dependent: :destroy</tt> or
      #   <tt>dependent: :nullify</tt> to override this.
      # [collection.destroy(object, ...)]
      #   Removes one or more objects from the collection by running <tt>destroy</tt> on
      #   each record, regardless of any dependent option, ensuring callbacks are run.
      #
      #   If the <tt>:through</tt> option is used, then the join records are destroyed
      #   instead, not the objects themselves.
      # [collection=objects]
      #   Replaces the collections content by deleting and adding objects as appropriate. If the <tt>:through</tt>
      #   option is true callbacks in the join models are triggered except destroy callbacks, since deletion is
      #   direct.
      # [collection_singular_ids]
      #   Returns an array of the associated objects' ids
      # [collection_singular_ids=ids]
      #   Replace the collection with the objects identified by the primary keys in +ids+. This
      #   method loads the models and calls <tt>collection=</tt>. See above.
      # [collection.clear]
      #   Removes every object from the collection. This destroys the associated objects if they
      #   are associated with <tt>dependent: :destroy</tt>, deletes them directly from the
      #   database if <tt>dependent: :delete_all</tt>, otherwise sets their foreign keys to +NULL+.
      #   If the <tt>:through</tt> option is true no destroy callbacks are invoked on the join models.
      #   Join models are directly deleted.
      # [collection.empty?]
      #   Returns +true+ if there are no associated objects.
      # [collection.size]
      #   Returns the number of associated objects.
      # [collection.find(...)]
      #   Finds an associated object according to the same rules as <tt>ActiveRecord::Base.find</tt>.
      # [collection.exists?(...)]
      #   Checks whether an associated object with the given conditions exists.
      #   Uses the same rules as <tt>ActiveRecord::Base.exists?</tt>.
      # [collection.build(attributes = {}, ...)]
      #   Returns one or more new objects of the collection type that have been instantiated
      #   with +attributes+ and linked to this object through a foreign key, but have not yet
      #   been saved.
      # [collection.create(attributes = {})]
      #   Returns a new object of the collection type that has been instantiated
      #   with +attributes+, linked to this object through a foreign key, and that has already
      #   been saved (if it passed the validation). *Note*: This only works if the base model
      #   already exists in the DB, not if it is a new (unsaved) record!
      # [collection.create!(attributes = {})]
      #   Does the same as <tt>collection.create</tt>, but raises <tt>ActiveRecord::RecordInvalid</tt>
      #   if the record is invalid.
      #
      # === Example
      #
      # A <tt>Firm</tt> class declares <tt>has_many :clients</tt>, which will add:
      # * <tt>Firm#clients</tt> (similar to <tt>Client.where(firm_id: id)</tt>)
      # * <tt>Firm#clients<<</tt>
      # * <tt>Firm#clients.delete</tt>
      # * <tt>Firm#clients.destroy</tt>
      # * <tt>Firm#clients=</tt>
      # * <tt>Firm#client_ids</tt>
      # * <tt>Firm#client_ids=</tt>
      # * <tt>Firm#clients.clear</tt>
      # * <tt>Firm#clients.empty?</tt> (similar to <tt>firm.clients.size == 0</tt>)
      # * <tt>Firm#clients.size</tt> (similar to <tt>Client.count "firm_id = #{id}"</tt>)
      # * <tt>Firm#clients.find</tt> (similar to <tt>Client.where(firm_id: id).find(id)</tt>)
      # * <tt>Firm#clients.exists?(name: 'ACME')</tt> (similar to <tt>Client.exists?(name: 'ACME', firm_id: firm.id)</tt>)
      # * <tt>Firm#clients.build</tt> (similar to <tt>Client.new("firm_id" => id)</tt>)
      # * <tt>Firm#clients.create</tt> (similar to <tt>c = Client.new("firm_id" => id); c.save; c</tt>)
      # * <tt>Firm#clients.create!</tt> (similar to <tt>c = Client.new("firm_id" => id); c.save!</tt>)
      # The declaration can also include an +options+ hash to specialize the behavior of the association.
      #
      # === Scopes
      #
      # You can pass a second argument +scope+ as a callable (i.e. proc or
      # lambda) to retrieve a specific set of records or customize the generated
      # query when you access the associated collection.
      #
      # Scope examples:
      #   has_many :comments, -> { where(author_id: 1) }
      #   has_many :employees, -> { joins(:address) }
      #   has_many :posts, ->(post) { where("max_post_length > ?", post.length) }
      #
      # === Extensions
      #
      # The +extension+ argument allows you to pass a block into a has_many
      # association. This is useful for adding new finders, creators and other
      # factory-type methods to be used as part of the association.
      #
      # Extension examples:
      #   has_many :employees do
      #     def find_or_create_by_name(name)
      #       first_name, last_name = name.split(" ", 2)
      #       find_or_create_by(first_name: first_name, last_name: last_name)
      #     end
      #   end
      #
      # === Options
      # [:class_name]
      #   Specify the class name of the association. Use it only if that name can't be inferred
      #   from the association name. So <tt>has_many :products</tt> will by default be linked
      #   to the Product class, but if the real class name is SpecialProduct, you'll have to
      #   specify it with this option.
      # [:foreign_key]
      #   Specify the foreign key used for the association. By default this is guessed to be the name
      #   of this class in lower-case and "_id" suffixed. So a Person class that makes a +has_many+
      #   association will use "person_id" as the default <tt>:foreign_key</tt>.
      # [:foreign_type]
      #   Specify the column used to store the associated object's type, if this is a polymorphic
      #   association. By default this is guessed to be the name of the polymorphic association
      #   specified on "as" option with a "_type" suffix. So a class that defines a
      #   <tt>has_many :tags, as: :taggable</tt> association will use "taggable_type" as the
      #   default <tt>:foreign_type</tt>.
      # [:primary_key]
      #   Specify the name of the column to use as the primary key for the association. By default this is +id+.
      # [:dependent]
      #   Controls what happens to the associated objects when
      #   their owner is destroyed. Note that these are implemented as
      #   callbacks, and Rails executes callbacks in order. Therefore, other
      #   similar callbacks may affect the <tt>:dependent</tt> behavior, and the
      #   <tt>:dependent</tt> behavior may affect other callbacks.
      #
      #   * <tt>:destroy</tt> causes all the associated objects to also be destroyed.
      #   * <tt>:delete_all</tt> causes all the associated objects to be deleted directly from the database (so callbacks will not be executed).
      #   * <tt>:nullify</tt> causes the foreign keys to be set to +NULL+. Callbacks are not executed.
      #   * <tt>:restrict_with_exception</tt> causes an exception to be raised if there are any associated records.
      #   * <tt>:restrict_with_error</tt> causes an error to be added to the owner if there are any associated objects.
      #
      #   If using with the <tt>:through</tt> option, the association on the join model must be
      #   a +belongs_to+, and the records which get deleted are the join records, rather than
      #   the associated records.
      # [:counter_cache]
      #   This option can be used to configure a custom named <tt>:counter_cache.</tt> You only need this option,
      #   when you customized the name of your <tt>:counter_cache</tt> on the <tt>belongs_to</tt> association.
      # [:as]
      #   Specifies a polymorphic interface (See <tt>belongs_to</tt>).
      # [:through]
      #   Specifies an association through which to perform the query. This can be any other type
      #   of association, including other <tt>:through</tt> associations. Options for <tt>:class_name</tt>,
      #   <tt>:primary_key</tt> and <tt>:foreign_key</tt> are ignored, as the association uses the
      #   source reflection.
      #
      #   If the association on the join model is a +belongs_to+, the collection can be modified
      #   and the records on the <tt>:through</tt> model will be automatically created and removed
      #   as appropriate. Otherwise, the collection is read-only, so you should manipulate the
      #   <tt>:through</tt> association directly.
      #
      #   If you are going to modify the association (rather than just read from it), then it is
      #   a good idea to set the <tt>:inverse_of</tt> option on the source association on the
      #   join model. This allows associated records to be built which will automatically create
      #   the appropriate join model records when they are saved. (See the 'Association Join Models'
      #   section above.)
      # [:source]
      #   Specifies the source association name used by <tt>has_many :through</tt> queries.
      #   Only use it if the name cannot be inferred from the association.
      #   <tt>has_many :subscribers, through: :subscriptions</tt> will look for either <tt>:subscribers</tt> or
      #   <tt>:subscriber</tt> on Subscription, unless a <tt>:source</tt> is given.
      # [:source_type]
      #   Specifies type of the source association used by <tt>has_many :through</tt> queries where the source
      #   association is a polymorphic +belongs_to+.
      # [:validate]
      #   If +false+, don't validate the associated objects when saving the parent object. true by default.
      # [:autosave]
      #   If true, always save the associated objects or destroy them if marked for destruction,
      #   when saving the parent object. If false, never save or destroy the associated objects.
      #   By default, only save associated objects that are new records. This option is implemented as a
      #   +before_save+ callback. Because callbacks are run in the order they are defined, associated objects
      #   may need to be explicitly saved in any user-defined +before_save+ callbacks.
      #
      #   Note that <tt>accepts_nested_attributes_for</tt> sets <tt>:autosave</tt> to <tt>true</tt>.
      # [:inverse_of]
      #   Specifies the name of the <tt>belongs_to</tt> association on the associated object
      #   that is the inverse of this <tt>has_many</tt> association. Does not work in combination
      #   with <tt>:through</tt> or <tt>:as</tt> options.
      #   See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail.
      # [:extend]
      #   Specifies a module or array of modules that will be extended into the association object returned.
      #   Useful for defining methods on associations, especially when they should be shared between multiple
      #   association objects.
      #
      # Option examples:
      #   has_many :comments, -> { order "posted_on" }
      #   has_many :comments, -> { includes :author }
      #   has_many :people, -> { where(deleted: false).order("name") }, class_name: "Person"
      #   has_many :tracks, -> { order "position" }, dependent: :destroy
      #   has_many :comments, dependent: :nullify
      #   has_many :tags, as: :taggable
      #   has_many :reports, -> { readonly }
      #   has_many :subscribers, through: :subscriptions, source: :user
      def has_many(name, scope = nil, options = {}, &extension)
        reflection = Builder::HasMany.build(self, name, scope, options, &extension)
        Reflection.add_reflection self, name, reflection
      end

      # Specifies a one-to-one association with another class. This method should only be used
      # if the other class contains the foreign key. If the current class contains the foreign key,
      # then you should use +belongs_to+ instead. See also ActiveRecord::Associations::ClassMethods's overview
      # on when to use +has_one+ and when to use +belongs_to+.
      #
      # The following methods for retrieval and query of a single associated object will be added:
      #
      # +association+ is a placeholder for the symbol passed as the +name+ argument, so
      # <tt>has_one :manager</tt> would add among others <tt>manager.nil?</tt>.
      #
      # [association(force_reload = false)]
      #   Returns the associated object. +nil+ is returned if none is found.
      # [association=(associate)]
      #   Assigns the associate object, extracts the primary key, sets it as the foreign key,
      #   and saves the associate object. To avoid database inconsistencies, permanently deletes an existing
      #   associated object when assigning a new one, even if the new one isn't saved to database.
      # [build_association(attributes = {})]
      #   Returns a new object of the associated type that has been instantiated
      #   with +attributes+ and linked to this object through a foreign key, but has not
      #   yet been saved.
      # [create_association(attributes = {})]
      #   Returns a new object of the associated type that has been instantiated
      #   with +attributes+, linked to this object through a foreign key, and that
      #   has already been saved (if it passed the validation).
      # [create_association!(attributes = {})]
      #   Does the same as <tt>create_association</tt>, but raises <tt>ActiveRecord::RecordInvalid</tt>
      #   if the record is invalid.
      #
      # === Example
      #
      # An Account class declares <tt>has_one :beneficiary</tt>, which will add:
      # * <tt>Account#beneficiary</tt> (similar to <tt>Beneficiary.where(account_id: id).first</tt>)
      # * <tt>Account#beneficiary=(beneficiary)</tt> (similar to <tt>beneficiary.account_id = account.id; beneficiary.save</tt>)
      # * <tt>Account#build_beneficiary</tt> (similar to <tt>Beneficiary.new("account_id" => id)</tt>)
      # * <tt>Account#create_beneficiary</tt> (similar to <tt>b = Beneficiary.new("account_id" => id); b.save; b</tt>)
      # * <tt>Account#create_beneficiary!</tt> (similar to <tt>b = Beneficiary.new("account_id" => id); b.save!; b</tt>)
      #
      # === Scopes
      #
      # You can pass a second argument +scope+ as a callable (i.e. proc or
      # lambda) to retrieve a specific record or customize the generated query
      # when you access the associated object.
      #
      # Scope examples:
      #   has_one :author, -> { where(comment_id: 1) }
      #   has_one :employer, -> { joins(:company) }
      #   has_one :dob, ->(dob) { where("Date.new(2000, 01, 01) > ?", dob) }
      #
      # === Options
      #
      # The declaration can also include an +options+ hash to specialize the behavior of the association.
      #
      # Options are:
      # [:class_name]
      #   Specify the class name of the association. Use it only if that name can't be inferred
      #   from the association name. So <tt>has_one :manager</tt> will by default be linked to the Manager class, but
      #   if the real class name is Person, you'll have to specify it with this option.
      # [:dependent]
      #   Controls what happens to the associated object when
      #   its owner is destroyed:
      #
      #   * <tt>:destroy</tt> causes the associated object to also be destroyed
      #   * <tt>:delete</tt> causes the associated object to be deleted directly from the database (so callbacks will not execute)
      #   * <tt>:nullify</tt> causes the foreign key to be set to +NULL+. Callbacks are not executed.
      #   * <tt>:restrict_with_exception</tt> causes an exception to be raised if there is an associated record
      #   * <tt>:restrict_with_error</tt> causes an error to be added to the owner if there is an associated object
      #
      #   Note that <tt>:dependent</tt> option is ignored when using <tt>:through</tt> option.
      # [:foreign_key]
      #   Specify the foreign key used for the association. By default this is guessed to be the name
      #   of this class in lower-case and "_id" suffixed. So a Person class that makes a +has_one+ association
      #   will use "person_id" as the default <tt>:foreign_key</tt>.
      # [:foreign_type]
      #   Specify the column used to store the associated object's type, if this is a polymorphic
      #   association. By default this is guessed to be the name of the polymorphic association
      #   specified on "as" option with a "_type" suffix. So a class that defines a
      #   <tt>has_one :tag, as: :taggable</tt> association will use "taggable_type" as the
      #   default <tt>:foreign_type</tt>.
      # [:primary_key]
      #   Specify the method that returns the primary key used for the association. By default this is +id+.
      # [:as]
      #   Specifies a polymorphic interface (See <tt>belongs_to</tt>).
      # [:through]
      #   Specifies a Join Model through which to perform the query. Options for <tt>:class_name</tt>,
      #   <tt>:primary_key</tt>, and <tt>:foreign_key</tt> are ignored, as the association uses the
      #   source reflection. You can only use a <tt>:through</tt> query through a <tt>has_one</tt>
      #   or <tt>belongs_to</tt> association on the join model.
      # [:source]
      #   Specifies the source association name used by <tt>has_one :through</tt> queries.
      #   Only use it if the name cannot be inferred from the association.
      #   <tt>has_one :favorite, through: :favorites</tt> will look for a
      #   <tt>:favorite</tt> on Favorite, unless a <tt>:source</tt> is given.
      # [:source_type]
      #   Specifies type of the source association used by <tt>has_one :through</tt> queries where the source
      #   association is a polymorphic +belongs_to+.
      # [:validate]
      #   If +false+, don't validate the associated object when saving the parent object. +false+ by default.
      # [:autosave]
      #   If true, always save the associated object or destroy it if marked for destruction,
      #   when saving the parent object. If false, never save or destroy the associated object.
      #   By default, only save the associated object if it's a new record.
      #
      #   Note that <tt>accepts_nested_attributes_for</tt> sets <tt>:autosave</tt> to <tt>true</tt>.
      # [:inverse_of]
      #   Specifies the name of the <tt>belongs_to</tt> association on the associated object
      #   that is the inverse of this <tt>has_one</tt> association. Does not work in combination
      #   with <tt>:through</tt> or <tt>:as</tt> options.
      #   See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail.
      # [:required]
      #   When set to +true+, the association will also have its presence validated.
      #   This will validate the association itself, not the id. You can use
      #   +:inverse_of+ to avoid an extra query during validation.
      #
      # Option examples:
      #   has_one :credit_card, dependent: :destroy  # destroys the associated credit card
      #   has_one :credit_card, dependent: :nullify  # updates the associated records foreign
      #                                                 # key value to NULL rather than destroying it
      #   has_one :last_comment, -> { order 'posted_on' }, class_name: "Comment"
      #   has_one :project_manager, -> { where role: 'project_manager' }, class_name: "Person"
      #   has_one :attachment, as: :attachable
      #   has_one :boss, -> { readonly }
      #   has_one :club, through: :membership
      #   has_one :primary_address, -> { where primary: true }, through: :addressables, source: :addressable
      #   has_one :credit_card, required: true
      def has_one(name, scope = nil, options = {})
        reflection = Builder::HasOne.build(self, name, scope, options)
        Reflection.add_reflection self, name, reflection
      end

      # Specifies a one-to-one association with another class. This method should only be used
      # if this class contains the foreign key. If the other class contains the foreign key,
      # then you should use +has_one+ instead. See also ActiveRecord::Associations::ClassMethods's overview
      # on when to use +has_one+ and when to use +belongs_to+.
      #
      # Methods will be added for retrieval and query for a single associated object, for which
      # this object holds an id:
      #
      # +association+ is a placeholder for the symbol passed as the +name+ argument, so
      # <tt>belongs_to :author</tt> would add among others <tt>author.nil?</tt>.
      #
      # [association(force_reload = false)]
      #   Returns the associated object. +nil+ is returned if none is found.
      # [association=(associate)]
      #   Assigns the associate object, extracts the primary key, and sets it as the foreign key.
      # [build_association(attributes = {})]
      #   Returns a new object of the associated type that has been instantiated
      #   with +attributes+ and linked to this object through a foreign key, but has not yet been saved.
      # [create_association(attributes = {})]
      #   Returns a new object of the associated type that has been instantiated
      #   with +attributes+, linked to this object through a foreign key, and that
      #   has already been saved (if it passed the validation).
      # [create_association!(attributes = {})]
      #   Does the same as <tt>create_association</tt>, but raises <tt>ActiveRecord::RecordInvalid</tt>
      #   if the record is invalid.
      #
      # === Example
      #
      # A Post class declares <tt>belongs_to :author</tt>, which will add:
      # * <tt>Post#author</tt> (similar to <tt>Author.find(author_id)</tt>)
      # * <tt>Post#author=(author)</tt> (similar to <tt>post.author_id = author.id</tt>)
      # * <tt>Post#build_author</tt> (similar to <tt>post.author = Author.new</tt>)
      # * <tt>Post#create_author</tt> (similar to <tt>post.author = Author.new; post.author.save; post.author</tt>)
      # * <tt>Post#create_author!</tt> (similar to <tt>post.author = Author.new; post.author.save!; post.author</tt>)
      # The declaration can also include an +options+ hash to specialize the behavior of the association.
      #
      # === Scopes
      #
      # You can pass a second argument +scope+ as a callable (i.e. proc or
      # lambda) to retrieve a specific record or customize the generated query
      # when you access the associated object.
      #
      # Scope examples:
      #   belongs_to :firm, -> { where(id: 2) }
      #   belongs_to :user, -> { joins(:friends) }
      #   belongs_to :level, ->(level) { where("game_level > ?", level.current) }
      #
      # === Options
      #
      # [:class_name]
      #   Specify the class name of the association. Use it only if that name can't be inferred
      #   from the association name. So <tt>belongs_to :author</tt> will by default be linked to the Author class, but
      #   if the real class name is Person, you'll have to specify it with this option.
      # [:foreign_key]
      #   Specify the foreign key used for the association. By default this is guessed to be the name
      #   of the association with an "_id" suffix. So a class that defines a <tt>belongs_to :person</tt>
      #   association will use "person_id" as the default <tt>:foreign_key</tt>. Similarly,
      #   <tt>belongs_to :favorite_person, class_name: "Person"</tt> will use a foreign key
      #   of "favorite_person_id".
      # [:foreign_type]
      #   Specify the column used to store the associated object's type, if this is a polymorphic
      #   association. By default this is guessed to be the name of the association with a "_type"
      #   suffix. So a class that defines a <tt>belongs_to :taggable, polymorphic: true</tt>
      #   association will use "taggable_type" as the default <tt>:foreign_type</tt>.
      # [:primary_key]
      #   Specify the method that returns the primary key of associated object used for the association.
      #   By default this is id.
      # [:dependent]
      #   If set to <tt>:destroy</tt>, the associated object is destroyed when this object is. If set to
      #   <tt>:delete</tt>, the associated object is deleted *without* calling its destroy method.
      #   This option should not be specified when <tt>belongs_to</tt> is used in conjunction with
      #   a <tt>has_many</tt> relationship on another class because of the potential to leave
      #   orphaned records behind.
      # [:counter_cache]
      #   Caches the number of belonging objects on the associate class through the use of +increment_counter+
      #   and +decrement_counter+. The counter cache is incremented when an object of this
      #   class is created and decremented when it's destroyed. This requires that a column
      #   named <tt>#{table_name}_count</tt> (such as +comments_count+ for a belonging Comment class)
      #   is used on the associate class (such as a Post class) - that is the migration for
      #   <tt>#{table_name}_count</tt> is created on the associate class (such that <tt>Post.comments_count</tt> will
      #   return the count cached, see note below). You can also specify a custom counter
      #   cache column by providing a column name instead of a +true+/+false+ value to this
      #   option (e.g., <tt>counter_cache: :my_custom_counter</tt>.)
      #   Note: Specifying a counter cache will add it to that model's list of readonly attributes
      #   using +attr_readonly+.
      # [:polymorphic]
      #   Specify this association is a polymorphic association by passing +true+.
      #   Note: If you've enabled the counter cache, then you may want to add the counter cache attribute
      #   to the +attr_readonly+ list in the associated classes (e.g. <tt>class Post; attr_readonly :comments_count; end</tt>).
      # [:validate]
      #   If +false+, don't validate the associated objects when saving the parent object. +false+ by default.
      # [:autosave]
      #   If true, always save the associated object or destroy it if marked for destruction, when
      #   saving the parent object.
      #   If false, never save or destroy the associated object.
      #   By default, only save the associated object if it's a new record.
      #
      #   Note that <tt>accepts_nested_attributes_for</tt> sets <tt>:autosave</tt> to <tt>true</tt>.
      # [:touch]
      #   If true, the associated object will be touched (the updated_at/on attributes set to current time)
      #   when this record is either saved or destroyed. If you specify a symbol, that attribute
      #   will be updated with the current time in addition to the updated_at/on attribute.
      # [:inverse_of]
      #   Specifies the name of the <tt>has_one</tt> or <tt>has_many</tt> association on the associated
      #   object that is the inverse of this <tt>belongs_to</tt> association. Does not work in
      #   combination with the <tt>:polymorphic</tt> options.
      #   See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail.
      # [:optional]
      #   When set to +true+, the association will not have its presence validated.
      # [:required]
      #   When set to +true+, the association will also have its presence validated.
      #   This will validate the association itself, not the id. You can use
      #   +:inverse_of+ to avoid an extra query during validation.
      #   NOTE: <tt>required</tt> is set to <tt>true</tt> by default and is deprecated. If
      #   you don't want to have association presence validated, use <tt>optional: true</tt>.
      #
      # Option examples:
      #   belongs_to :firm, foreign_key: "client_of"
      #   belongs_to :person, primary_key: "name", foreign_key: "person_name"
      #   belongs_to :author, class_name: "Person", foreign_key: "author_id"
      #   belongs_to :valid_coupon, ->(o) { where "discounts > ?", o.payments_count },
      #                             class_name: "Coupon", foreign_key: "coupon_id"
      #   belongs_to :attachable, polymorphic: true
      #   belongs_to :project, -> { readonly }
      #   belongs_to :post, counter_cache: true
      #   belongs_to :comment, touch: true
      #   belongs_to :company, touch: :employees_last_updated_at
      #   belongs_to :user, optional: true
      def belongs_to(name, scope = nil, options = {})
        reflection = Builder::BelongsTo.build(self, name, scope, options)
        Reflection.add_reflection self, name, reflection
      end

      # Specifies a many-to-many relationship with another class. This associates two classes via an
      # intermediate join table. Unless the join table is explicitly specified as an option, it is
      # guessed using the lexical order of the class names. So a join between Developer and Project
      # will give the default join table name of "developers_projects" because "D" precedes "P" alphabetically.
      # Note that this precedence is calculated using the <tt><</tt> operator for String. This
      # means that if the strings are of different lengths, and the strings are equal when compared
      # up to the shortest length, then the longer string is considered of higher
      # lexical precedence than the shorter one. For example, one would expect the tables "paper_boxes" and "papers"
      # to generate a join table name of "papers_paper_boxes" because of the length of the name "paper_boxes",
      # but it in fact generates a join table name of "paper_boxes_papers". Be aware of this caveat, and use the
      # custom <tt>:join_table</tt> option if you need to.
      # If your tables share a common prefix, it will only appear once at the beginning. For example,
      # the tables "catalog_categories" and "catalog_products" generate a join table name of "catalog_categories_products".
      #
      # The join table should not have a primary key or a model associated with it. You must manually generate the
      # join table with a migration such as this:
      #
      #   class CreateDevelopersProjectsJoinTable < ActiveRecord::Migration
      #     def change
      #       create_join_table :developers, :projects
      #     end
      #   end
      #
      # It's also a good idea to add indexes to each of those columns to speed up the joins process.
      # However, in MySQL it is advised to add a compound index for both of the columns as MySQL only
      # uses one index per table during the lookup.
      #
      # Adds the following methods for retrieval and query:
      #
      # +collection+ is a placeholder for the symbol passed as the +name+ argument, so
      # <tt>has_and_belongs_to_many :categories</tt> would add among others <tt>categories.empty?</tt>.
      #
      # [collection(force_reload = false)]
      #   Returns an array of all the associated objects.
      #   An empty array is returned if none are found.
      # [collection<<(object, ...)]
      #   Adds one or more objects to the collection by creating associations in the join table
      #   (<tt>collection.push</tt> and <tt>collection.concat</tt> are aliases to this method).
      #   Note that this operation instantly fires update SQL without waiting for the save or update call on the
      #   parent object, unless the parent object is a new record.
      # [collection.delete(object, ...)]
      #   Removes one or more objects from the collection by removing their associations from the join table.
      #   This does not destroy the objects.
      # [collection.destroy(object, ...)]
      #   Removes one or more objects from the collection by running destroy on each association in the join table, overriding any dependent option.
      #   This does not destroy the objects.
      # [collection=objects]
      #   Replaces the collection's content by deleting and adding objects as appropriate.
      # [collection_singular_ids]
      #   Returns an array of the associated objects' ids.
      # [collection_singular_ids=ids]
      #   Replace the collection by the objects identified by the primary keys in +ids+.
      # [collection.clear]
      #   Removes every object from the collection. This does not destroy the objects.
      # [collection.empty?]
      #   Returns +true+ if there are no associated objects.
      # [collection.size]
      #   Returns the number of associated objects.
      # [collection.find(id)]
      #   Finds an associated object responding to the +id+ and that
      #   meets the condition that it has to be associated with this object.
      #   Uses the same rules as <tt>ActiveRecord::Base.find</tt>.
      # [collection.exists?(...)]
      #   Checks whether an associated object with the given conditions exists.
      #   Uses the same rules as <tt>ActiveRecord::Base.exists?</tt>.
      # [collection.build(attributes = {})]
      #   Returns a new object of the collection type that has been instantiated
      #   with +attributes+ and linked to this object through the join table, but has not yet been saved.
      # [collection.create(attributes = {})]
      #   Returns a new object of the collection type that has been instantiated
      #   with +attributes+, linked to this object through the join table, and that has already been
      #   saved (if it passed the validation).
      #
      # === Example
      #
      # A Developer class declares <tt>has_and_belongs_to_many :projects</tt>, which will add:
      # * <tt>Developer#projects</tt>
      # * <tt>Developer#projects<<</tt>
      # * <tt>Developer#projects.delete</tt>
      # * <tt>Developer#projects.destroy</tt>
      # * <tt>Developer#projects=</tt>
      # * <tt>Developer#project_ids</tt>
      # * <tt>Developer#project_ids=</tt>
      # * <tt>Developer#projects.clear</tt>
      # * <tt>Developer#projects.empty?</tt>
      # * <tt>Developer#projects.size</tt>
      # * <tt>Developer#projects.find(id)</tt>
      # * <tt>Developer#projects.exists?(...)</tt>
      # * <tt>Developer#projects.build</tt> (similar to <tt>Project.new("developer_id" => id)</tt>)
      # * <tt>Developer#projects.create</tt> (similar to <tt>c = Project.new("developer_id" => id); c.save; c</tt>)
      # The declaration may include an +options+ hash to specialize the behavior of the association.
      #
      # === Scopes
      #
      # You can pass a second argument +scope+ as a callable (i.e. proc or
      # lambda) to retrieve a specific set of records or customize the generated
      # query when you access the associated collection.
      #
      # Scope examples:
      #   has_and_belongs_to_many :projects, -> { includes :milestones, :manager }
      #   has_and_belongs_to_many :categories, ->(category) {
      #     where("default_category = ?", category.name)
      #   }
      #
      # === Extensions
      #
      # The +extension+ argument allows you to pass a block into a
      # has_and_belongs_to_many association. This is useful for adding new
      # finders, creators and other factory-type methods to be used as part of
      # the association.
      #
      # Extension examples:
      #   has_and_belongs_to_many :contractors do
      #     def find_or_create_by_name(name)
      #       first_name, last_name = name.split(" ", 2)
      #       find_or_create_by(first_name: first_name, last_name: last_name)
      #     end
      #   end
      #
      # === Options
      #
      # [:class_name]
      #   Specify the class name of the association. Use it only if that name can't be inferred
      #   from the association name. So <tt>has_and_belongs_to_many :projects</tt> will by default be linked to the
      #   Project class, but if the real class name is SuperProject, you'll have to specify it with this option.
      # [:join_table]
      #   Specify the name of the join table if the default based on lexical order isn't what you want.
      #   <b>WARNING:</b> If you're overwriting the table name of either class, the +table_name+ method
      #   MUST be declared underneath any +has_and_belongs_to_many+ declaration in order to work.
      # [:foreign_key]
      #   Specify the foreign key used for the association. By default this is guessed to be the name
      #   of this class in lower-case and "_id" suffixed. So a Person class that makes
      #   a +has_and_belongs_to_many+ association to Project will use "person_id" as the
      #   default <tt>:foreign_key</tt>.
      # [:association_foreign_key]
      #   Specify the foreign key used for the association on the receiving side of the association.
      #   By default this is guessed to be the name of the associated class in lower-case and "_id" suffixed.
      #   So if a Person class makes a +has_and_belongs_to_many+ association to Project,
      #   the association will use "project_id" as the default <tt>:association_foreign_key</tt>.
      # [:readonly]
      #   If true, all the associated objects are readonly through the association.
      # [:validate]
      #   If +false+, don't validate the associated objects when saving the parent object. +true+ by default.
      # [:autosave]
      #   If true, always save the associated objects or destroy them if marked for destruction, when
      #   saving the parent object.
      #   If false, never save or destroy the associated objects.
      #   By default, only save associated objects that are new records.
      #
      #   Note that <tt>accepts_nested_attributes_for</tt> sets <tt>:autosave</tt> to <tt>true</tt>.
      #
      # Option examples:
      #   has_and_belongs_to_many :projects
      #   has_and_belongs_to_many :projects, -> { includes :milestones, :manager }
      #   has_and_belongs_to_many :nations, class_name: "Country"
      #   has_and_belongs_to_many :categories, join_table: "prods_cats"
      #   has_and_belongs_to_many :categories, -> { readonly }
      def has_and_belongs_to_many(name, scope = nil, options = {}, &extension)
        if scope.is_a?(Hash)
          options = scope
          scope   = nil
        end

        habtm_reflection = ActiveRecord::Reflection::HasAndBelongsToManyReflection.new(name, scope, options, self)

        builder = Builder::HasAndBelongsToMany.new name, self, options

        join_model = builder.through_model

        const_set join_model.name, join_model
        private_constant join_model.name

        middle_reflection = builder.middle_reflection join_model

        Builder::HasMany.define_callbacks self, middle_reflection
        Reflection.add_reflection self, middle_reflection.name, middle_reflection
        middle_reflection.parent_reflection = habtm_reflection

        include Module.new {
          class_eval <<-RUBY, __FILE__, __LINE__ + 1
          def destroy_associations
            association(:#{middle_reflection.name}).delete_all(:delete_all)
            association(:#{name}).reset
            super
          end
          RUBY
        }

        hm_options = {}
        hm_options[:through] = middle_reflection.name
        hm_options[:source] = join_model.right_reflection.name

        [:before_add, :after_add, :before_remove, :after_remove, :autosave, :validate, :join_table, :class_name, :extend].each do |k|
          hm_options[k] = options[k] if options.key? k
        end

        has_many name, scope, hm_options, &extension
        self._reflections[name.to_s].parent_reflection = habtm_reflection
      end
    end
  end
end