title: How to display associated model's attribute in Active Admin index with belongs_to/has_many relationship (Rails 3.2/Active Admin)
body: <p>I'm building a daily deal Rails app to learn RoR.</p>
  
  <p>I am facing a problem for the past few hours : i can't get a model's attribute of an other associated model on active admin. Let me show you exactly the problem :</p>
  
  <p>I have two models: Brand (i.e the brand of the deal) and Deal. A deal belongs to a Brand but a Brand can have many Deals.</p>
  
  <p>models/deal.rb is like this:</p>
  
  <pre><code>class Deal &lt; ActiveRecord::Base
    belongs_to :brand
  </code></pre>
  
  <p>and we have models/brand.rb:</p>
  
  <pre><code>class Brand &lt; ActiveRecord::Base    
    has_many :deals
  
    attr_accessible :name
  </code></pre>
  
  <p>And i did the t.belongs_to in my migrations so this is ok.</p>
  
  <p>In Active Admin's Deals' create form , i type, as admin, which brand the deal is associated with:</p>
  
  <p>admin/game.rb</p>
  
  <pre><code>ActiveAdmin.register Deal do
  # -- Form -----------------------------------------------------------
    form do |f|
      f.inputs "Brand (i.e. client)" do
        f.input :brand_id, :label =&gt; "Select a brand:", :as =&gt; :select, :collection =&gt; Brand.all
      end
  </code></pre>
  
  <p>it works great, and i can create Deals with a certain brand.
  but <strong>I CAN'T manage to display the NAME of the Brand in my list of Deals</strong>:</p>
  
  <pre><code>ActiveAdmin.register Deal do
  index do   
  selectable_column   
  # id_column 
  column :title
  column :deal_amount
  column :brand do |deal|
    link_to deal.brand.name
  end
  </code></pre>
  
  <p>...doesn't work.</p>
  
  <p>How can I do that ?</p>
  
  <p>I tried everything but <strong>i basically don't know how to fetch the name of a Brand given it matches the brand_id in the Deal's table</strong>.</p>
  
  <p>Any help appreciated.</p>