title: Is it possible to have a join table with only two primary keys when using Rails ActiveAdmin?
body: <p>I can edit a join table in ActiveAdmin, only if the join table has one, and only one, primary key (id).  </p>
  
  <p>If I try to design a join table with only two fields (primary key on both), ActiveAdmin returns a NilClass error when I attempt to add a new record : <code>undefined method 'humanize' for nil:NilClass</code>.</p>
  
  <p>model :</p>
  
  <pre><code>class IngredientProduct &lt; ActiveRecord::Base
    belongs_to :product
    belongs_to :ingredient
  end
  
  class Ingredient &lt; ActiveRecord::Base
  end
  
  class Product &lt; ActiveRecord::Base
  end
  </code></pre>
  
  <p>ingredient_products fields :</p>
  
  <pre><code>product_id --&gt; primary key, nn, integer
  ingredient_id --&gt; primary key, nn, integer
  </code></pre>
  
  <p>That doesn't work.  My table "ingredient_products" must be designed this way :</p>
  
  <pre><code>id --&gt; primary key, nn, integer
  product_id --&gt; integer
  ingredient_id --&gt; integer
  </code></pre>
  
  <p>to allow me to edit or add record with ActiveAdmin.  </p>
  
  <p>Do you know if ActiveAdmin support join tables that have only two fields, both used to define the primary key ?</p>
  
  <p>My project : MySql, Rails 4.0.1 with ActiveRecord and ActiveAdmin 1.0.0.pre</p>