Implements the details of eager loading of Active Record associations. Application developers should not use this module directly.
ActiveRecord::Base
is extended with this module. The source
code in ActiveRecord::Base
references methods defined in this
module.
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.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.
See also ActiveRecord::Associations::ClassMethods, which explains eager loading in a more high-level (application developer-friendly) manner.
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
.
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 } ]
preload_options
contains options that will be passed to
ActiveRecord::Base#find (which is called under the hood for preloading
records). But it is passed only one level deep in the
associations
argument, i.e. it's not passed to the child
associations when associations
is a Hash.
# File lib/active_record/association_preload.rb, line 87 def preload_associations(records, associations, preload_options={}) records = Array.wrap(records).compact return if records.empty? case associations when Array then associations.each {|association| preload_associations(records, association, preload_options)} when Symbol, String then preload_one_association(records, associations.to_sym, preload_options) when Hash then associations.each do |parent, child| raise "parent must be an association name" unless parent.is_a?(String) || parent.is_a?(Symbol) preload_associations(records, parent, preload_options) reflection = reflections[parent] parents = records.sum { |record| Array.wrap(record.send(reflection.name)) } unless parents.empty? parents = parents.uniq if reflection.macro == :belongs_to parents.first.class.preload_associations(parents, child) end end end end