Class: ActiveRecord::Associations::Preloader

Inherits:
Object
  • Object
show all
Extended by:
ActiveSupport::Autoload
Defined in:
lib/active_record/associations/preloader.rb,
lib/active_record/associations/preloader/association.rb,
lib/active_record/associations/preloader/through_association.rb

Overview

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.

Defined Under Namespace

Classes: AlreadyLoaded, Association, ThroughAssociation

Instance Method Summary collapse

Instance Method Details

#preload(records, associations, preload_scope = nil) ⇒ Object

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 } ]



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# File 'lib/active_record/associations/preloader.rb', line 85

def preload(records, associations, preload_scope = nil)
  records = Array.wrap(records).compact

  if records.empty?
    []
  else
    records.uniq!
    Array.wrap(associations).flat_map { |association|
      preloaders_on association, records, preload_scope
    }
  end
end