docs: def add_field(self, field):
          """Add the received field to the model."""
          self.remove_field(field.name)
          self._fields[field.name] = field
  
          if field.default is not None:
              if six.callable(field.default):
                  self._default_callables[field.key] = field.default
              else:
                  self._defaults[field.key] = field.default