title: rolify+activeAdmin add role by form
body: <p>i m using rolify+activeadmin gems.
  I have 2 resource: Staff and User (default devise table).
  Staff is a model that map an only read table, so i can't write in staffs table.
  i'm trying with active admin to add a role for a user using has_one and belongs_to associations:</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    rolify
    belongs_to :staff
  end
  
  class Staff &lt; ActiveRecord::Base
    has_one :user
  end
  </code></pre>
  
  <p>in the app/admin/staff.rb class i have this:</p>
  
  <pre><code>    form do |f|
        f.inputs "Add role" do  |staff|
          f.input :roles,  :as =&gt; :select,      :collection =&gt; Role.global
        end
        f.actions
      end
  
  So i want to add a role for a user using Staff admin resource.
  when i click on submit form button i have this error:
  NoMethodError in Admin/staffs#edit
  
  Showing app/views/active_admin/resource/edit.html.arb where line #1 raised:
  
  undefined method `roles' for #&lt;Staff:0x00000005c6af70&gt;
  Extracted source (around line #1):
  
  1: insert_tag renderer_for(:edit)
  </code></pre>