title: Mongoid - Use relation field in where query
body: <p>I have this models:</p>
  
  <pre><code>class Document1
    field privacy
    has_many events, :as =&gt; :target
  end
  
  class Document2
    field privacy
    has many_events, :as =&gt; :target
  end
  
  class Event
    belongs_to :target, :polymorphic =&gt; true
  end
  </code></pre>
  
  <p>I need to do something like:</p>
  
  <pre><code>Event.where( :target.privacy =&gt; :public )
  </code></pre>
  
  <p>How could I do it ?</p>