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Class: ActiveRecord::Associations::Preloader

Do not use. This class is for internal use only.

Overview

Active Record Preloader

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 Method Summary

::ActiveSupport::Autoload - Extended

autoload, autoload_at, autoload_under, eager_autoload, eager_load!

Instance Attribute Summary

Instance Method Summary

Constructor Details

.new(records:, associations:, scope: nil, available_records: [], associate_by_default: true) ⇒ Preloader

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 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 their 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 arguments to QueryMethods#includes. So #associations could look like this:

:books
[ :books, :author ]
{ author: :avatar }
[ :books, { author: :avatar } ]

available_records is an array of ::ActiveRecord::Base. The Preloader will try to use the objects in this array to preload the requested associations before querying the database. This can save database queries by reusing in-memory objects. The optimization is only applied to single associations (i.e. :belongs_to, :has_one) with no scopes.

[ GitHub ] 

  


# File 'activerecord/lib/active_record/associations/preloader.rb', line 99



def initialize(records:, associations:, scope: nil, available_records: [], associate_by_default: true)
  @records = records
  @associations = associations
  @scope = scope
  @available_records = available_records || []
  @associate_by_default = associate_by_default

  @tree = Branch.new(
    parent: nil,
    association: nil,
    children: @associations,
    associate_by_default: @associate_by_default,
    scope: @scope
  )
  @tree.preloaded_records = @records
end


  







Instance Attribute Details



  

    #associate_by_default   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/associations/preloader.rb', line 56



attr_reader :records, :associations, :scope, :associate_by_default


  








  

    #associations   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/associations/preloader.rb', line 56



attr_reader :records, :associations, :scope, :associate_by_default


  








  

    #empty?  ⇒ Boolean  (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/associations/preloader.rb', line 116



def empty?
  associations.nil? || records.length == 0
end


  








  

    #records   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/associations/preloader.rb', line 56



attr_reader :records, :associations, :scope, :associate_by_default


  








  

    #scope   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/associations/preloader.rb', line 56



attr_reader :records, :associations, :scope, :associate_by_default


  







Instance Method Details



  

    #branches  
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/associations/preloader.rb', line 126



def branches
  @tree.children
end


  








  

    #call  
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/associations/preloader.rb', line 120



def call
  Batch.new([self], available_records: @available_records).call

  loaders
end


  








  

    #loaders  
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/associations/preloader.rb', line 130



def loaders
  branches.flat_map(&:loaders)
end