title: Rails 4: Order has_many in serializer
body: <p>I have a serializer with a <code>has_many</code> association and I want to order the dependent model.  For some reason I get an <code>undefined method key?'</code> error:</p>
  
  <pre><code>class API::ClientSerializer &lt; ActiveModel::Serializer
      has_many :check_ins, -&gt; { order(:week) }
  end
  </code></pre>
  
  <p><strong>How else do I order the check_ins by week?</strong></p>