aggregations.rb

module ActiveRecord
  module Aggregations # :nodoc:
    def self.append_features(base)
      super
      base.extend(ClassMethods)
    end

    # Active Record implements aggregation through a macro-like class method called +composed_of+ for representing attributes 
    # as value objects. It expresses relationships like "Account [is] composed of Money [among other things]" or "Person [is]
    # composed of [an] address". Each call to the macro adds a description on how the value objects are created from the 
    # attributes of the entity object (when the entity is initialized either as a new object or from finding an existing) 
    # and how it can be turned back into attributes (when the entity is saved to the database). Example:
    #
    #   class Customer < ActiveRecord::Base
    #     composed_of :balance, :class_name => "Money", :mapping => %w(balance amount)
    #     composed_of :address, :mapping => [ %w(address_street street), %w(address_city city) ]
    #   end
    #
    # The customer class now has the following methods to manipulate the value objects:
    # * <tt>Customer#balance, Customer#balance=(money)</tt>
    # * <tt>Customer#address, Customer#address=(address)</tt>
    #
    # These methods will operate with value objects like the ones described below:
    #
    #  class Money
    #    include Comparable
    #    attr_reader :amount, :currency
    #    EXCHANGE_RATES = { "USD_TO_DKK" => 6 }  
    # 
    #    def initialize(amount, currency = "USD") 
    #      @amount, @currency = amount, currency 
    #    end
    #
    #    def exchange_to(other_currency)
    #      exchanged_amount = (amount * EXCHANGE_RATES["#{currency}_TO_#{other_currency}"]).floor
    #      Money.new(exchanged_amount, other_currency)
    #    end
    #
    #    def ==(other_money)
    #      amount == other_money.amount && currency == other_money.currency
    #    end
    #
    #    def <=>(other_money)
    #      if currency == other_money.currency
    #        amount <=> amount
    #      else
    #        amount <=> other_money.exchange_to(currency).amount
    #      end
    #    end
    #  end
    #
    #  class Address
    #    attr_reader :street, :city
    #    def initialize(street, city) 
    #      @street, @city = street, city 
    #    end
    #
    #    def close_to?(other_address) 
    #      city == other_address.city 
    #    end
    #
    #    def ==(other_address)
    #      city == other_address.city && street == other_address.street
    #    end
    #  end
    #  
    # Now it's possible to access attributes from the database through the value objects instead. If you choose to name the
    # composition the same as the attributes name, it will be the only way to access that attribute. That's the case with our
    # +balance+ attribute. You interact with the value objects just like you would any other attribute, though:
    #
    #   customer.balance = Money.new(20)     # sets the Money value object and the attribute
    #   customer.balance                     # => Money value object
    #   customer.balance.exchanged_to("DKK") # => Money.new(120, "DKK")
    #   customer.balance > Money.new(10)     # => true
    #   customer.balance == Money.new(20)    # => true
    #   customer.balance < Money.new(5)      # => false
    #
    # Value objects can also be composed of multiple attributes, such as the case of Address. The order of the mappings will
    # determine the order of the parameters. Example:
    #
    #   customer.address_street = "Hyancintvej"
    #   customer.address_city   = "Copenhagen"
    #   customer.address        # => Address.new("Hyancintvej", "Copenhagen")
    #   customer.address = Address.new("May Street", "Chicago")
    #   customer.address_street # => "May Street" 
    #   customer.address_city   # => "Chicago" 
    #
    # == Writing value objects
    #
    # Value objects are immutable and interchangeable objects that represent a given value, such as a Money object representing
    # $5. Two Money objects both representing $5 should be equal (through methods such == and <=> from Comparable if ranking makes
    # sense). This is unlike a entity objects where equality is determined by identity. An entity class such as Customer can
    # easily have two different objects that both have an address on Hyancintvej. Entity identity is determined by object or
    # relational unique identifiers (such as primary keys). Normal ActiveRecord::Base classes are entity objects.
    #
    # It's also important to treat the value objects as immutable. Don't allow the Money object to have its amount changed after
    # creation. Create a new money object with the new value instead. This is examplified by the Money#exchanged_to method that
    # returns a new value object instead of changing its own values. Active Record won't persist value objects that have been
    # changed through other means than the writer method.
    #
    # The immutable requirement is enforced by Active Record by freezing any object assigned as a value object. Attempting to 
    # change it afterwards will result in a TypeError.
    # 
    # Read more about value objects on http://c2.com/cgi/wiki?ValueObject and on the dangers of not keeping value objects
    # immutable on http://c2.com/cgi/wiki?ValueObjectsShouldBeImmutable
    module ClassMethods
      # Adds the a reader and writer method for manipulating a value object, so
      # <tt>composed_of :address</tt> would add <tt>address</tt> and <tt>address=(new_address)</tt>.
      #
      # Options are:
      # * <tt>:class_name</tt>  - specify the class name of the association. Use it only if that name can't be infered
      #   from the part id. So <tt>composed_of :address</tt> will by default be linked to the +Address+ class, but
      #   if the real class name is +CompanyAddress+, you'll have to specify it with this option.
      # * <tt>:mapping</tt> - specifies a number of mapping arrays (attribute, parameter) that bind an attribute name
      #   to a constructor parameter on the value class.
      #
      # Option examples:
      #   composed_of :temperature, :mapping => %w(reading celsius)
      #   composed_of :balance, :class_name => "Money", :mapping => %w(balance amount)
      #   composed_of :address, :mapping => [ %w(address_street street), %w(address_city city) ]
      def composed_of(part_id, options = {})
        validate_options([ :class_name, :mapping ], options.keys)

        name        = part_id.id2name
        class_name  = options[:class_name] || name_to_class_name(name)
        mapping     = options[:mapping]

        reader_method(name, class_name, mapping)
        writer_method(name, class_name, mapping)
      end

      private
        # Raises an exception if an invalid option has been specified to prevent misspellings from slipping through 
        def validate_options(valid_option_keys, supplied_option_keys)
          unknown_option_keys = supplied_option_keys - valid_option_keys
          raise(ActiveRecordError, "Unknown options: #{unknown_option_keys}") unless unknown_option_keys.empty?
        end

        def name_to_class_name(name)
          name.capitalize.gsub(/_(.)/) { |s| $1.capitalize }
        end
        
        def reader_method(name, class_name, mapping)
          module_eval <<-end_eval
            def #{name}(force_reload = false)
              if @#{name}.nil? || force_reload
                @#{name} = #{class_name}.new(#{(Array === mapping.first ? mapping : [ mapping ]).collect{ |pair| "read_attribute(\"#{pair.first}\")"}.join(", ")})
              end
              
              return @#{name}
            end
          end_eval
        end        
        
        def writer_method(name, class_name, mapping)
          module_eval <<-end_eval
            def #{name}=(part)
              @#{name} = part.freeze
              #{(Array === mapping.first ? mapping : [ mapping ]).collect{ |pair| "@attributes[\"#{pair.first}\"] = part.#{pair.last}" }.join("\n")}
            end
          end_eval
        end
    end
  end
end