'A Bokeh protocol "reference" to this model, i.e. a dict of the
form:
.. code-block:: python
\'type\' : << view model name >>
\'id\'   : << unique model id >>
Additionally there may be a `subtype` field if this model is a subtype.'

@property
def ref(self):

    if ('__subtype__' in self.__class__.__dict__):
        return {'type': self.__view_model__, 'subtype': self.__subtype__, 'id': self._id}
    else:
        return {'type': self.__view_model__, 'id': self._id}
