Check if the command to be executed is a modifier, to connect the object.
        Then call _traverse_command.

def _call_command(self, name, *args, **kwargs):
        """
        Check if the command to be executed is a modifier, to connect the object.
        Then call _traverse_command.
        """
        obj = getattr(self, '_instance', self)  # _instance if a field, self if an instance

        # The object may not be already connected, so if we want to update a
        # field, connect it before.
        # If the object as no PK yet, let the object create itself
        if name in self.available_modifiers and obj._pk and not obj.connected:
            obj.connect()

        # Give priority to a "_call_{commmand}" method
        meth = getattr(self, '_call_%s' % name, self._traverse_command)
        return meth(name, *args, **kwargs)