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module ActiveRecord
module Associations
# Implements the details of eager loading of Active Record associations.
# Note that 'eager loading' and 'preloading' are actually the same thing.
# However, there are two different eager loading strategies.
# The first one is by using table joins. This was only strategy available
# prior to Rails 2.1. Suppose that you have an Author model with columns
# 'name' and 'age', and a Book model with columns 'name' and 'sales'. Using
# this strategy, Active Record would try to retrieve all data for an author
# and all of its books via a single query:
# SELECT * FROM authors
# LEFT OUTER JOIN books ON = books.author_id
# WHERE = 'Ken Akamatsu'
# However, this could result in many rows that contain redundant data. After
# having received the first row, we already have enough data to instantiate
# the Author object. In all subsequent rows, only the data for the joined
# 'books' table is useful; the joined 'authors' data is just redundant, and
# processing this redundant data takes memory and CPU time. The problem
# quickly becomes worse and worse as the level of eager loading increases
# (i.e. if Active Record is to eager load the associations' associations as
# well).
# The second strategy is to use multiple database queries, one for each
# level of association. Since Rails 2.1, this is the default strategy. In
# situations where a table join is necessary (e.g. when the +:conditions+
# option references an association's column), it will fallback to the table
# join strategy.
class Preloader #:nodoc:
autoload :Association, 'active_record/associations/preloader/association'
autoload :SingularAssociation, 'active_record/associations/preloader/singular_association'
autoload :CollectionAssociation, 'active_record/associations/preloader/collection_association'
autoload :ThroughAssociation, 'active_record/associations/preloader/through_association'
autoload :HasMany, 'active_record/associations/preloader/has_many'
autoload :HasManyThrough, 'active_record/associations/preloader/has_many_through'
autoload :HasOne, 'active_record/associations/preloader/has_one'
autoload :HasOneThrough, 'active_record/associations/preloader/has_one_through'
autoload :HasAndBelongsToMany, 'active_record/associations/preloader/has_and_belongs_to_many'
autoload :BelongsTo, 'active_record/associations/preloader/belongs_to'
attr_reader :records, :associations, :options, :model
# Eager loads the named associations for the given Active Record record(s).
# In this description, 'association name' shall refer to the name passed
# to an association creation method. For example, a model that specifies
# <tt>belongs_to :author</tt>, <tt>has_many :buyers</tt> has association
# names +:author+ and +:buyers+.
# == Parameters
# +records+ is an array of ActiveRecord::Base. This array needs not be flat,
# i.e. +records+ itself may also contain arrays of records. In any case,
# +preload_associations+ will preload the all associations records by
# flattening +records+.
# +associations+ specifies one or more associations that you want to
# preload. It may be:
# - a Symbol or a String which specifies a single association name. For
# example, specifying +:books+ allows this method to preload all books
# for an Author.
# - an Array which specifies multiple association names. This array
# is processed recursively. For example, specifying <tt>[:avatar, :books]</tt>
# allows this method to preload an author's avatar as well as all of his
# books.
# - a Hash which specifies multiple association names, as well as
# association names for the to-be-preloaded association objects. For
# example, specifying <tt>{ :author => :avatar }</tt> will preload a
# book's author, as well as that author's avatar.
# +:associations+ has the same format as the +:include+ option for
# <tt>ActiveRecord::Base.find</tt>. So +associations+ could look like this:
# :books
# [ :books, :author ]
# { :author => :avatar }
# [ :books, { :author => :avatar } ]
# +options+ contains options that will be passed to ActiveRecord::Base#find
# (which is called under the hood for preloading records). But it is passed
# only one level deep in the +associations+ argument, i.e. it's not passed
# to the child associations when +associations+ is a Hash.
def initialize(records, associations, options = {})
@records = Array.wrap(records).compact.uniq
@associations = Array.wrap(associations)
@options = options
def run
unless records.empty?
associations.each { |association| preload(association) }
def preload(association)
case association
when Hash
when String, Symbol
raise ArgumentError, "#{association.inspect} was not recognised for preload"
def preload_hash(association)
association.each do |parent, child|, parent, options).run { |record| record.send(parent) }.flatten, child).run
# Not all records have the same class, so group then preload group on the reflection
# itself so that if various subclass share the same association then we do not split
# them unnecessarily
# Additionally, polymorphic belongs_to associations can have multiple associated
# classes, depending on the polymorphic_type field. So we group by the classes as
# well.
def preload_one(association)
grouped_records(association).each do |reflection, klasses|
klasses.each do |klass, records|
preloader_for(reflection).new(klass, records, reflection, options).run
def grouped_records(association)
records_by_reflection(association).map do |reflection, records|
[reflection, records.group_by { |record| association_klass(reflection, record) }]
def records_by_reflection(association)
records.group_by do |record|
reflection = record.class.reflections[association]
unless reflection
raise ActiveRecord::ConfigurationError, "Association named '#{association}' was not found; " \
"perhaps you misspelled it?"
def association_klass(reflection, record)
if reflection.macro == :belongs_to && reflection.options[:polymorphic]
klass = record.send(reflection.foreign_type)
klass && klass.constantize
def preloader_for(reflection)
case reflection.macro
when :has_many
reflection.options[:through] ? HasManyThrough : HasMany
when :has_one
reflection.options[:through] ? HasOneThrough : HasOne
when :has_and_belongs_to_many
when :belongs_to