diff options
Diffstat (limited to 'activerecord/lib/active_record/associations/preloader.rb')
| -rw-r--r-- | activerecord/lib/active_record/associations/preloader.rb | 169 |
1 files changed, 98 insertions, 71 deletions
diff --git a/activerecord/lib/active_record/associations/preloader.rb b/activerecord/lib/active_record/associations/preloader.rb index 0848e7afb3..4358f3b581 100644 --- a/activerecord/lib/active_record/associations/preloader.rb +++ b/activerecord/lib/active_record/associations/preloader.rb @@ -2,33 +2,42 @@ 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. + # Suppose that you have the following two Active Record models: # - # 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: + # class Author < ActiveRecord::Base + # # columns: name, age + # has_many :books + # end # - # SELECT * FROM authors - # LEFT OUTER JOIN books ON authors.id = books.author_id - # WHERE authors.name = 'Ken Akamatsu' + # class Book < ActiveRecord::Base + # # columns: title, sales + # end # - # 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). + # 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. # - # 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 @@ -42,12 +51,9 @@ module ActiveRecord 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' end - attr_reader :records, :associations, :preload_scope, :model - # Eager loads the named associations for the given Active Record record(s). # # In this description, 'association name' shall refer to the name passed @@ -82,36 +88,48 @@ module ActiveRecord # [ :books, :author ] # { author: :avatar } # [ :books, { author: :avatar } ] - def initialize(records, associations, preload_scope = nil) - @records = Array.wrap(records).compact.uniq - @associations = Array.wrap(associations) - @preload_scope = preload_scope || Relation.new(nil, nil) - end - def run - unless records.empty? - associations.each { |association| preload(association) } + 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 preload(association) + def preloaders_on(association, records, scope) case association when Hash - preload_hash(association) - when String, Symbol - preload_one(association.to_sym) + 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 preload_hash(association) - association.each do |parent, child| - Preloader.new(records, parent, preload_scope).run - Preloader.new(records.map { |record| record.send(parent) }.flatten, child).run - 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 # Not all records have the same class, so group then preload group on the reflection @@ -121,52 +139,61 @@ module ActiveRecord # 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, preload_scope).run + 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) - Hash[ - records_by_reflection(association).map do |reflection, records| - [reflection, records.group_by { |record| association_klass(reflection, record) }] - end - ] + def grouped_records(association, records) + h = {} + records.each do |record| + next unless record + assoc = record.association(association) + klasses = h[assoc.reflection] ||= {} + (klasses[assoc.klass] ||= []) << record + end + h end - def records_by_reflection(association) - records.group_by do |record| - reflection = record.class.reflections[association] + class AlreadyLoaded + attr_reader :owners, :reflection - unless reflection - raise ActiveRecord::ConfigurationError, "Association named '#{association}' was not found; " \ - "perhaps you misspelled it?" - end + def initialize(klass, owners, reflection, preload_scope) + @owners = owners + @reflection = reflection + end + + def run(preloader); end - reflection + def preloaded_records + owners.flat_map { |owner| owner.association(reflection.name).target } end end - def association_klass(reflection, record) - if reflection.macro == :belongs_to && reflection.options[:polymorphic] - klass = record.send(reflection.foreign_type) - klass && klass.constantize - else - reflection.klass - end + class NullPreloader + def self.new(klass, owners, reflection, preload_scope); self; end + def self.run(preloader); end + def self.preloaded_records; []; end end - def preloader_for(reflection) + def preloader_for(reflection, owners, rhs_klass) + return NullPreloader unless rhs_klass + + 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 :has_and_belongs_to_many - HasAndBelongsToMany when :belongs_to BelongsTo end |