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 authors.id = books.author_id # WHERE authors.name = '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: extend ActiveSupport::Autoload eager_autoload do 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 :BelongsTo, 'active_record/associations/preloader/belongs_to' end # 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 # belongs_to :author, has_many :buyers 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 [:avatar, :books] # 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 { author: :avatar } will preload a # book's author, as well as that author's avatar. # # +:associations+ has the same format as the +:include+ option for # ActiveRecord::Base.find. So +associations+ could look like this: # # :books # [ :books, :author ] # { author: :avatar } # [ :books, { author: :avatar } ] NULL_RELATION = Struct.new(:values, :bind_values).new({}, []) def preload(records, associations, preload_scope = nil) records = Array.wrap(records).compact.uniq associations = Array.wrap(associations) preload_scope = preload_scope || NULL_RELATION if records.empty? [] else associations.flat_map { |association| preloaders_on association, records, preload_scope } end end private def preloaders_on(association, records, scope) case association when Hash preloaders_for_hash(association, records, scope) when Symbol preloaders_for_one(association, records, scope) when String preloaders_for_one(association.to_sym, records, scope) else raise ArgumentError, "#{association.inspect} was not recognised for preload" end end def preloaders_for_hash(association, records, scope) parent, child = association.to_a.first # hash should only be of length 1 loaders = preloaders_for_one parent, records, scope recs = loaders.flat_map(&:preloaded_records).uniq loaders.concat Array.wrap(child).flat_map { |assoc| preloaders_on assoc, recs, scope } end # 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 preloaders_for_one(association, records, scope) grouped_records(association, records).flat_map do |reflection, klasses| klasses.map do |rhs_klass, rs| loader = preloader_for(reflection, rs, rhs_klass).new(rhs_klass, rs, reflection, scope) loader.run self loader end end end def grouped_records(association, records) h = {} records.each do |record| assoc = record.association(association) klasses = h[assoc.reflection] ||= {} (klasses[assoc.klass] ||= []) << record end h end class AlreadyLoaded attr_reader :owners, :reflection def initialize(klass, owners, reflection, preload_scope) @owners = owners @reflection = reflection end def run(preloader); end def preloaded_records owners.flat_map { |owner| owner.association(reflection.name).target } end end class NullPreloader def self.new(klass, owners, reflection, preload_scope); self; end def self.run(preloader); end end def preloader_for(reflection, owners, rhs_klass) return NullPreloader unless rhs_klass if owners.first.association(reflection.name).loaded? return AlreadyLoaded end case reflection.macro when :has_many reflection.options[:through] ? HasManyThrough : HasMany when :has_one reflection.options[:through] ? HasOneThrough : HasOne when :belongs_to BelongsTo end end end end end