title: has_many :through in fixtures in rails 3.1
body: <p>I want to set up some has_many, :through objects using fixtures for testing in rails 3.1.</p>
  
  <p>If you look at the documentation on the association as an example, I want to know how to assign assemblies to parts. The only way I've found to do it is to explicitly create the manifest object in it's own file, but if I'm happy with the defaults on that model and all I want to specify are the part/assembly id's then is there a way to do it directly?</p>
  
  <p><a href="http://guides.rubyonrails.org/association_basics.html#choosing-between-has_many-through-and-has_and_belongs_to_many">http://guides.rubyonrails.org/association_basics.html#choosing-between-has_many-through-and-has_and_belongs_to_many</a></p>
  
  <p>I want something like:</p>
  
  <pre><code>my_assembly:
      parts: my_first_part, my_second_part
  </code></pre>
  
  <p>This works if you do HABTM, but not when you have the explicit join model.</p>