diff options
Diffstat (limited to 'activerecord/lib/active_record/associations/preloader.rb')
| -rw-r--r-- | activerecord/lib/active_record/associations/preloader.rb | 177 |
1 files changed, 177 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/associations/preloader.rb b/activerecord/lib/active_record/associations/preloader.rb new file mode 100644 index 0000000000..fafed94ff2 --- /dev/null +++ b/activerecord/lib/active_record/associations/preloader.rb @@ -0,0 +1,177 @@ +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.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: + 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 + end + + def run + unless records.empty? + associations.each { |association| preload(association) } + end + end + + private + + def preload(association) + case association + when Hash + preload_hash(association) + when String, Symbol + preload_one(association.to_sym) + 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, options).run + Preloader.new(records.map { |record| record.send(parent) }.flatten, child).run + end + 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 preload_one(association) + grouped_records(association).each do |reflection, klasses| + klasses.each do |klass, records| + preloader_for(reflection).new(klass, records, reflection, options).run + 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 + ] + end + + 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?" + end + + reflection + 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 + end + + 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 + HasAndBelongsToMany + when :belongs_to + BelongsTo + end + end + end + end +end |