module ActiveRecord
module Associations
# Implements the details of eager loading of Active Record associations.
#
# Suppose that you have the following two Active Record models:
#
# class Author < ActiveRecord::Base
# # columns: name, age
# has_many :books
# end
#
# class Book < ActiveRecord::Base
# # columns: title, sales, author_id
# end
#
# When you load an author with all associated books Active Record will make
# multiple queries like this:
#
# Author.includes(:books).where(name: ['bell hooks', 'Homer']).to_a
#
# => SELECT `authors`.* FROM `authors` WHERE `name` IN ('bell hooks', 'Homer')
# => SELECT `books`.* FROM `books` WHERE `author_id` IN (2, 5)
#
# Active Record saves the ids of the records from the first query to use in
# the second. Depending on the number of associations involved there can be
# arbitrarily many SQL queries made.
#
# However, if there is a WHERE clause that spans across tables Active
# Record will fall back to a slightly more resource-intensive single query:
#
# Author.includes(:books).where(books: {title: 'Illiad'}).to_a
# => SELECT `authors`.`id` AS t0_r0, `authors`.`name` AS t0_r1, `authors`.`age` AS t0_r2,
# `books`.`id` AS t1_r0, `books`.`title` AS t1_r1, `books`.`sales` AS t1_r2
# FROM `authors`
# LEFT OUTER JOIN `books` ON `authors`.`id` = `books`.`author_id`
# WHERE `books`.`title` = 'Illiad'
#
# This could result in many rows that contain redundant data and it performs poorly at scale
# and is therefore only used when necessary.
#
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
NULL_RELATION = Struct.new(:values, :where_clause, :joins_values).new({}, Relation::WhereClause.empty, [])
# 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 } ]
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
# Loads all the given data into +records+ for the +association+.
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 recognized for preload"
end
end
def preloaders_for_hash(association, records, scope)
association.flat_map { |parent, child|
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
}
loaders
}
end
# Loads all the given data into +records+ for a singular +association+.
#
# Functions by instantiating a preloader class such as Preloader::HasManyThrough and
# call the +run+ method for each passed in class in the +records+ argument.
#
# 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).new(rhs_klass, rs, reflection, scope)
loader.run self
loader
end
end
end
def grouped_records(association, records)
h = {}
records.each do |record|
next unless record
assoc = record.association(association)
next unless assoc.klass
klasses = h[assoc.reflection] ||= {}
(klasses[assoc.klass] ||= []) << record
end
h
end
class AlreadyLoaded # :nodoc:
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
# Returns a class containing the logic needed to load preload the data
# and attach it to a relation. For example +Preloader::Association+ or
# +Preloader::HasManyThrough+. The class returned implements a `run` method
# that accepts a preloader.
def preloader_for(reflection, owners)
if owners.first.association(reflection.name).loaded?
return AlreadyLoaded
end
reflection.check_preloadable!
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
