title: rails rabl multiple collections
body: <p>I'm having a bit of trouble with <code>rabl</code>. This code </p>
  
  <pre><code>collection favourite_groups, root: :groups
  
  extends 'favourites/base'
  </code></pre>
  
  <p>gives me the following json structure: <code>{ groups: [ {..}, {..} ] }</code>. What I need is to append another node that contains an array - <code>{ groups: [ {..}, {..} ], users: [ {..}, {..} ] }</code> The data is in separate variables, but it can be in one, which ever makes this work.</p>
  
  <p>Ideas?</p>