title: Rails 3.2. find by not equal polymorphic association type
body: <p>I have a User model with </p>
  
  <pre><code>belongs_to :role, polymorphic: true
  </code></pre>
  
  <p>When I query by role_type</p>
  
  <pre><code>(rdb:1) User.count
  14
  (rdb:1) User.where(role_type: 'Student').count
  3
  (rdb:1) User.where('role_type != ?', 'Student').count
  0
  </code></pre>
  
  <p>No matter what I do, I can't match on not equal in this column.  Other columns, I can.</p>