:meth:`add_field` must be used to add a field to an existing
        instance of Model. This method is required so that serialization of the
        data is possible. Data on existing fields (defined in the class) can be
        reassigned without using this method.

def add_field(self, key, value, field):
        ''':meth:`add_field` must be used to add a field to an existing
        instance of Model. This method is required so that serialization of the
        data is possible. Data on existing fields (defined in the class) can be
        reassigned without using this method.

        '''
        self._extra[key] = field
        setattr(self, key, value)