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/batch.rb,
lib/active_record/associations/preloader/branch.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: Association, Batch, Branch, ThroughAssociation

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(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 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 method in ActiveRecord::QueryMethods. 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.


96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
# File 'lib/active_record/associations/preloader.rb', line 96

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_defaultObject (readonly)

Returns the value of attribute associate_by_default.


54
55
56
# File 'lib/active_record/associations/preloader.rb', line 54

def associate_by_default
  @associate_by_default
end

#associationsObject (readonly)

Returns the value of attribute associations.


54
55
56
# File 'lib/active_record/associations/preloader.rb', line 54

def associations
  @associations
end

#recordsObject (readonly)

Returns the value of attribute records.


54
55
56
# File 'lib/active_record/associations/preloader.rb', line 54

def records
  @records
end

#scopeObject (readonly)

Returns the value of attribute scope.


54
55
56
# File 'lib/active_record/associations/preloader.rb', line 54

def scope
  @scope
end

Instance Method Details

#branchesObject


123
124
125
# File 'lib/active_record/associations/preloader.rb', line 123

def branches
  @tree.children
end

#callObject


117
118
119
120
121
# File 'lib/active_record/associations/preloader.rb', line 117

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

  loaders
end

#empty?Boolean

Returns:

  • (Boolean)

113
114
115
# File 'lib/active_record/associations/preloader.rb', line 113

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

#loadersObject


127
128
129
# File 'lib/active_record/associations/preloader.rb', line 127

def loaders
  branches.flat_map(&:loaders)
end