ActiveRecord::Associations::ClassMethods

Associations are a set of macro-like class methods for tying objects together through foreign keys. They express relationships like "Project has one Project Manager" or "Project belongs to a Portfolio". Each macro adds a number of methods to the class which are specialized according to the collection or association symbol and the options hash. It works much the same way as Ruby’s own attr* methods. Example:

  class Project < ActiveRecord::Base
    belongs_to              :portfolio
    has_one                 :project_manager
    has_many                :milestones
    has_and_belongs_to_many :categories
  end

The project class now has the following methods (and more) to ease the traversal and manipulation of its relationships:

• Project#portfolio, Project#portfolio=(portfolio), Project#portfolio.nil?
• Project#project_manager, Project#project_manager=(project_manager), Project#project_manager.nil?,
• Project#milestones.empty?, Project#milestones.size, Project#milestones, Project#milestones<<(milestone), Project#milestones.delete(milestone), Project#milestones.find(milestone_id), Project#milestones.find_all(conditions), Project#milestones.build, Project#milestones.create
• Project#categories.empty?, Project#categories.size, Project#categories, Project#categories<<(category1), Project#categories.delete(category1)

Example

files/examples/associations.png

Is it belongs_to or has_one?

Both express a 1-1 relationship, the difference is mostly where to place the foreign key, which goes on the table for the class saying belongs_to. Example:

  class Post < ActiveRecord::Base
    has_one :author
  end

  class Author < ActiveRecord::Base
    belongs_to :post
  end

The tables for these classes could look something like:

  CREATE TABLE posts (
    id int(11) NOT NULL auto_increment,
    title varchar default NULL,
    PRIMARY KEY  (id)
  )

  CREATE TABLE authors (
    id int(11) NOT NULL auto_increment,
    post_id int(11) default NULL,
    name varchar default NULL,
    PRIMARY KEY  (id)
  )

Unsaved objects and associations

You can manipulate objects and associations before they are saved to the database, but there is some special behaviour you should be aware of, mostly involving the saving of associated objects.

One-to-one associations

• Assigning an object to a has_one association automatically saves that object and the object being replaced (if there is one), in order to update their primary keys - except if the parent object is unsaved (new_record? == true).
• If either of these saves fail (due to one of the objects being invalid) the assignment statement returns false and the assignment is cancelled.
• If you wish to assign an object to a has_one association without saving it, use the #association.build method (documented below).
• Assigning an object to a belongs_to association does not save the object, since the foreign key field belongs on the parent. It does not save the parent either.

Collections

• Adding an object to a collection (has_many or has_and_belongs_to_many) automatically saves that object, except if the parent object (the owner of the collection) is not yet stored in the database.
• If saving any of the objects being added to a collection (via #push or similar) fails, then #push returns false.
• You can add an object to a collection without automatically saving it by using the #collection.build method (documented below).
• All unsaved (new_record? == true) members of the collection are automatically saved when the parent is saved.

Association callbacks

Similiar to the normal callbacks that hook into the lifecycle of an Active Record object, you can also define callbacks that get trigged when you add an object to or removing an object from a association collection. Example:

  class Project
    has_and_belongs_to_many :developers, :after_add => :evaluate_velocity

    def evaluate_velocity(developer)
      ...
    end
  end

It’s possible to stack callbacks by passing them as an array. Example:

  class Project
    has_and_belongs_to_many :developers, :after_add => [:evaluate_velocity, Proc.new { |p, d| p.shipping_date = Time.now}]
  end

Possible callbacks are: before_add, after_add, before_remove and after_remove.

Should any of the before_add callbacks throw an exception, the object does not get added to the collection. Same with the before_remove callbacks, if an exception is thrown the object doesn’t get removed.

Association extensions

The proxy objects that controls the access to associations can be extended through anonymous modules. This is especially beneficial for adding new finders, creators, and other factory-type methods that are only used as part of this associatio. Example:

  class Account < ActiveRecord::Base
    has_many :people do
      def find_or_create_by_name(name)
        first_name, *last_name = name.split
        last_name = last_name.join " "

        find_or_create_by_first_name_and_last_name(first_name, last_name)
      end
    end
  end

  person = Account.find(:first).people.find_or_create_by_name("David Heinemeier Hansson")
  person.first_name # => "David"
  person.last_name  # => "Heinemeier Hansson"

If you need to share the same extensions between many associations, you can use a named extension module. Example:

  module FindOrCreateByNameExtension
    def find_or_create_by_name(name)
      first_name, *last_name = name.split
      last_name = last_name.join " "

      find_or_create_by_first_name_and_last_name(first_name, last_name)
    end
  end

  class Account < ActiveRecord::Base
    has_many :people, :extend => FindOrCreateByNameExtension
  end

  class Company < ActiveRecord::Base
    has_many :people, :extend => FindOrCreateByNameExtension
  end

Caching

All of the methods are built on a simple caching principle that will keep the result of the last query around unless specifically instructed not to. The cache is even shared across methods to make it even cheaper to use the macro-added methods without worrying too much about performance at the first go. Example:

  project.milestones             # fetches milestones from the database
  project.milestones.size        # uses the milestone cache
  project.milestones.empty?      # uses the milestone cache
  project.milestones(true).size  # fetches milestones from the database
  project.milestones             # uses the milestone cache

Eager loading of associations

Eager loading is a way to find objects of a certain class and a number of named associations along with it in a single SQL call. This is one of the easiest ways of to prevent the dreaded 1+N problem in which fetching 100 posts that each needs to display their author triggers 101 database queries. Through the use of eager loading, the 101 queries can be reduced to 1. Example:

  class Post < ActiveRecord::Base
    belongs_to :author
    has_many   :comments
  end

Consider the following loop using the class above:

  for post in Post.find(:all)
    puts "Post:            " + post.title
    puts "Written by:      " + post.author.name
    puts "Last comment on: " + post.comments.first.created_on
  end

To iterate over these one hundred posts, we’ll generate 201 database queries. Let’s first just optimize it for retrieving the author:

  for post in Post.find(:all, :include => :author)

This references the name of the belongs_to association that also used the :author symbol, so the find will now weave in a join something like this: LEFT OUTER JOIN authors ON authors.id = posts.author_id. Doing so will cut down the number of queries from 201 to 101.

We can improve upon the situation further by referencing both associations in the finder with:

  for post in Post.find(:all, :include => [ :author, :comments ])

That’ll add another join along the lines of: LEFT OUTER JOIN comments ON comments.post_id = posts.id. And we’ll be down to 1 query. But that shouldn’t fool you to think that you can pull out huge amounts of data with no performance penalty just because you’ve reduced the number of queries. The database still needs to send all the data to Active Record and it still needs to be processed. So it’s no catch-all for performance problems, but it’s a great way to cut down on the number of queries in a situation as the one described above.

Please note that limited eager loading with has_many and has_and_belongs_to_many associations is not compatible with describing conditions on these eager tables. This will work:

  Post.find(:all, :include => :comments, :conditions => "posts.title = 'magic forest'", :limit => 2)

…but this will not (and an ArgumentError will be raised):

  Post.find(:all, :include => :comments, :conditions => "comments.body like 'Normal%'", :limit => 2)

Also have in mind that since the eager loading is pulling from multiple tables, you’ll have to disambiguate any column references in both conditions and orders. So :order => "posts.id DESC" will work while :order => "id DESC" will not. This may require that you alter the :order and :conditions on the association definitions themselves.

It’s currently not possible to use eager loading on multiple associations from the same table. Eager loading will also not pull additional attributes on join tables, so "rich associations" with has_and_belongs_to_many is not a good fit for eager loading.

Modules

By default, associations will look for objects within the current module scope. Consider:

  module MyApplication
    module Business
      class Firm < ActiveRecord::Base
         has_many :clients
       end

      class Company < ActiveRecord::Base; end
    end
  end

When Firm#clients is called, it’ll in turn call MyApplication::Business::Company.find(firm.id). If you want to associate with a class in another module scope this can be done by specifying the complete class name, such as:

  module MyApplication
    module Business
      class Firm < ActiveRecord::Base; end
    end

    module Billing
      class Account < ActiveRecord::Base
        belongs_to :firm, :class_name => "MyApplication::Business::Firm"
      end
    end
  end

Type safety with ActiveRecord::AssociationTypeMismatch

If you attempt to assign an object to an association that doesn’t match the inferred or specified :class_name, you’ll get a ActiveRecord::AssociationTypeMismatch.

Options

All of the association macros can be specialized through options which makes more complex cases than the simple and guessable ones possible.

activerecord/lib/active_record/associations.rb
activerecord/lib/active_record/deprecated_associations.rb