<p>Setting up polymorphic associations is ridiculously easy in Rails. Setting up many-to-many associations in Rails is <em>also</em> ridiculously easy in Rails. However, setting up polymorphic many-to-many associations in Rails is more difficult, but only slightly.</p>
<p>Recently, on an <a href="http://www.intridea.com">Intridea</a> client project, I had one particular model that had many-to-many associations with several other models in the app. The thought of multiple join tables in the database didn't sit well with me, so I decided to consolidate things a bit.</p>
<p>Now I can't use the actual models from the app without possibly giving away the yet-to-be-launched app. So instead, I'll use an example that we're all familiar with, tags and taggings.</p>
<p>You know the drill. You have your common model, Tag.</p>
<pre name="code" class="ruby">class Tag < ActiveRecord::Base
has_many :taggings, :dependent => :destroy
has_many :books, :through => :tagings, :source => :taggable, :source_type => "Book"
has_many :movies, :through => :tagings, :source => :taggable, :source_type => "Movie"
end
</pre>
<p>Your join model, Tagging.</p>
<pre name="code" class="ruby">class Tagging < ActiveRecord::Base
belongs_to :taggable, :polymorphic => true
belongs_to :tag
end
</pre>
<p>And your taggable models, Book and Movie.</p>
<pre name="code" class="ruby">class Book < ActiveRecord::Base
has_many :taggings, :as => :taggable
has_many :tags, :through => :taggings
end
class Movie < ActiveRecord::Base
has_many :taggings, :as => :taggable
has_many :tags, :through => :taggings
end
</pre>
<p>Everything in Tagging, Movie, and Book is pretty much your standard setup. This gives you all the normal associations. You've got tag.taggings, book.tags, movie.tags, and tag.taggable.</p>
<p>It gets a little trickier when you want to find all movies (or books) associated with a tag. There are a number of ways that this can be done. You could use some crazy joined finder call on Movie, or find all Taggings with a particular taggable type (or a named scope) and collect the movies, or do it like up above.</p>
<pre name="code" class="ruby">has_many :movies, :through => :tagings,
:source => :taggable,
:source_type => "Movie"
</pre>
<p>This association allows us to access a movies with a particular tag directly from the tag object itself. It's just your typical <em>has_many</em> association, but with a few more options. The <em>:through</em> option says that movies can be accessed through our join model, tagging. Rails normally would expect there to be a movies association on tagging, but the <em>:source</em> option tells Rails to examine the taggable association instead. Similarly, the <em>:source_type</em> option specifies the class (or type) of the polymorphic association that we trying to retrieve.</p>
<p>So remember when you want bidirectional class-specific many-to-many polymorphic associations, :source and :source_type are your friend.</p>
2009-04-12T17:38:26Z
Setting up polymorphic associations is ridiculously easy in Rails.? Setting up many-to-many associations in Rails is also ridiculously easy in Rails.?
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polymorphic-many-to-many-associations-in-rails
Polymorphic many-to-many associations in Rails
2009-05-04T04:05:23Z