title: Inverse (write_only) SerializerMethodField in Django REST Framework
body: <p>In the Django REST Framework, is there an "inverse" of the <code>SerializerMethodField</code> which is <code>write_only</code>? What I am looking for is a <code>Serializer</code> that will set a field on the model based on the value returned by a method.</p>
  
  <p>I do not want this field to render a widget however.</p>