title: Undefined method 'on' for ActionModel
body: <p>I'm getting the following error:</p>
  
  <pre><code>NoMethodError in Users#new
  
  Showing .../app/views/users/form/_new.haml where line #7 raised:
  
  undefined method `on' for #&lt;ActiveModel::Errors:0x007fb599ec6610&gt;
  </code></pre>
  
  <p>The code in line 7 is:</p>
  
  <pre><code>7:   = errors_for user, :first_name
  </code></pre>
  
  <p>And the application_helper.rb:</p>
  
  <pre><code>def errors_for(model, field)
    error = case errors = model.errors.on(field)
    ...
  end
  </code></pre>
  
  <p>'on' is a <a href="http://ar.rubyonrails.org/classes/ActiveRecord/Errors.html#M000306">default method in ActiveRecord</a>. Why doesn't this work?</p>