title: In rails_admin, how can I filter based on the presence of an association id?
body: <p>In rails_admin, I've got a list of cities. Some have a numeric <code>state_id</code> and some have a nil. I want my cities list view to let me filter based on whether that field is filled or blank.</p>
  
  <p>How can I do that?</p>
  
  <p>This raises an exception:</p>
  
  <pre><code>config.model 'City' do
    list do
      filters [:state_id]
      ...
  </code></pre>
  
  <p>... because rails_admin can't find "city_id" among its list of "filterable fields", even though it's one of the displayed fields.</p>