Class: ActiveRecord::Associations::Preloader
- Extended by:
- ActiveSupport::Autoload
- Defined in:
- activerecord/lib/active_record/associations/preloader.rb,
activerecord/lib/active_record/associations/preloader/association.rb,
activerecord/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
-
#preload(records, associations, preload_scope = nil) ⇒ Object
Eager loads the named associations for the given Active Record record(s).
Methods included from ActiveSupport::Autoload
autoload, autoload_at, autoload_under, autoloads, eager_autoload, eager_load!, extended
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 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 'activerecord/lib/active_record/associations/preloader.rb', line 85 def preload(records, associations, preload_scope = nil) records = Array.wrap(records).compact if records.empty? [] else Array.wrap(associations).flat_map { |association| preloaders_on association, records, preload_scope } end end |