title: ActiveRecord::HasManyThroughNestedAssociationsAreReadonly Error in Rails Admin
body: <p>I just upgraded to <code>Rails 3.2.10</code> and am getting an error message that I never used to get when updating a record via RailsAdmin.</p>
  
  <pre><code>ActiveRecord::HasManyThroughNestedAssociationsAreReadonly at /admin/vendor/12/edit
  
  Message Cannot modify association 'Vendor#categories' because it goes through more than one other association.
  </code></pre>
  
  <p>This is my <code>Vendor</code> model:</p>
  
  <pre><code>class Vendor &lt; ActiveRecord::Base
    attr_accessible :name, :description, :banner_image, :logo_image, :intro_text, :thumb_image, :category_ids, :product_ids, :user_id, :remove_banner_image, :banner_image_cache, :remove_logo_image, :logo_image_cache
      mount_uploader :banner_image, ImageUploader
      mount_uploader :logo_image, ImageUploader
      mount_uploader :thumb_image, ImageUploader
  
      has_many :products, :dependent =&gt; :destroy
      has_many :categories, :through =&gt; :products
      belongs_to :owner, :class_name =&gt; "User",
          :foreign_key =&gt; "user_id"   
  end
  </code></pre>
  
  <p>This is my <code>Category</code> model:</p>
  
  <pre><code>class Category &lt; ActiveRecord::Base
    attr_accessible :name, :product_ids, :category_ids
      has_many :category_products do
           def with_products
             includes(:product)
           end
         end
  
    has_many :products, :through =&gt; :category_products
  
  end
  </code></pre>
  
  <p>This is my <code>Product</code> model:</p>
  
  <pre><code>class Product &lt; ActiveRecord::Base
    attr_accessible :name, :description, :price, :vendor_id, :image, :category_ids, :sku, :remove_image, :image_cache
      mount_uploader :image, ImageUploader
  
      belongs_to :vendor
      has_many :category_products do
             def with_categories
               includes(:category)
             end
      end
  
      has_many :categories, :through =&gt; :category_products
  
  end
  </code></pre>
  
  <p>This is my <code>CategoryProduct</code> model:</p>
  
  <pre><code>class CategoryProduct &lt; ActiveRecord::Base
    attr_accessible :product_id, :category_id, :purchases_count
  
      belongs_to :product
    belongs_to :category
  
    validates_uniqueness_of :product_id, :scope =&gt; :category_id
  end
  </code></pre>