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/has_one.rb,
lib/active_record/associations/preloader/has_many.rb,
lib/active_record/associations/preloader/belongs_to.rb,
lib/active_record/associations/preloader/association.rb,
lib/active_record/associations/preloader/has_one_through.rb,
lib/active_record/associations/preloader/has_many_through.rb,
lib/active_record/associations/preloader/through_association.rb,
lib/active_record/associations/preloader/singular_association.rb,
lib/active_record/associations/preloader/collection_association.rb

Overview

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.author_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.

Defined Under Namespace

Modules: ThroughAssociation Classes: AlreadyLoaded, Association, BelongsTo, CollectionAssociation, HasMany, HasManyThrough, HasOne, HasOneThrough, NullPreloader, SingularAssociation

Constant Summary collapse

NULL_RELATION =

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 } ]

Struct.new(:values).new({})

Instance Method Summary collapse

Instance Method Details

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



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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.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