title: undefined method `columns_hash'
body: <p>I have used <code>shoulda_matchers</code> in my <code>specs</code>. For this test</p>
  
  <pre><code>it { should validate_uniqueness_of(:name).scoped_to(:role_id) }
  </code></pre>
  
  <p>I get this error:</p>
  
  <pre><code>Expected errors to include "has already been taken" when name is set to "arbitrary_string", got errors: ["name is already taken (\"arbitrary_string\")"]
  </code></pre>
  
  <p>When I change the test to:</p>
  
  <pre><code>it { should validate_uniqueness_of(:name).scoped_to(:role_id).with_message('is already taken') }
  </code></pre>
  
  <p>I am getting this error:</p>
  
  <pre><code> NoMethodError:
         undefined method `columns_hash' for Activity:Class
  </code></pre>
  
  <p>My model has only one field - <code>name</code></p>