code: def bind_model_cls(self, model_cls):
          
          if self.model_cls:
              raise errors.Error('"{0}" has been already bound to "{1}" and '
                                 'could not be rebound to "{2}"'.format(
                                     self, self.model_cls, model_cls))
          self.model_cls = model_cls
          return self
docs: Bind field to model class.