title: defaults for to_json in Rails with :include
body: <p>Let us say I have  a model Post which belongs to a User. To convert to json, I do something like this</p>
  
  <pre><code>@reply.to_json(:include =&gt; {:user =&gt; {:only =&gt; [:email, :id]}, 
                 :only =&gt; [:title, :id])
  </code></pre>
  
  <p>However, I want to set some defaults for this so I don't have to specify :only everytime. I am trying to override as_json to accomplish this. When I add as_json in User model, it is called when I do @user.to_json but when user is included in @reply.to_json, my overriden as_json for User is ignored.</p>
  
  <p>How do I make this work?  </p>
  
  <p>Thanks</p>