title: How to cast a child model object to its base model in django-polymorphic?
body: <p>I have <code>MyBaseModel(PolymorphicModel)</code> and <code>MyChildModel(MyBaseModel)</code>. I retrieve instances of MyChildModel from DB and want to cast the to MyBaseModel in order to pass them to a generic serialisation function (I want them to be serialised as MyBaseModel).</p>
  
  <p>Is this possible?</p>
  
  <p>I tried <code>MyBaseModel(my_child_model_instance)</code>, but it produces new instance instead of casting existing.</p>