title: as_json not including association
body: <p>I am trying to override the <code>as_json</code> method in my <code>Edge</code> class so I can include the  <code>x</code> and <code>y</code> coordinates of it's from and to <code>Vertex</code>.</p>
  
  <p>The assosiations are like this:</p>
  
  <p><code>Edge belongs_to :from</code> and <code>:to</code> Vertex.</p>
  
  <p><code>Vertex has_many :edges</code>, where <code>:x</code> and <code>:y</code> are fields.</p>
  
  <p>I have tried several syntactic variation, like the one below, but can't get it to work. Thanks for any help!</p>
  
  <pre><code>def as_json(options={})
    super only: [:name, :value, :color], include: [
                                    { from: { only: [:x, :y] } },
                                    {   to: { only: [:x, :y] } } 
  end
  </code></pre>