Hook that replaces the `Field` attribute on a class with a named
        ``FieldDescriptor``. Called by the metaclass during construction of the
        ``Model``.

def add_to_class(self, cls, name):
        '''
        Hook that replaces the `Field` attribute on a class with a named
        ``FieldDescriptor``. Called by the metaclass during construction of the
        ``Model``.
        '''
        self._name = name
        self._container_model_class = cls
        setattr(cls, name, FieldDescriptor(self))
        self._bound = True