'Validates that we aren\'t trying to do a query like
value__in=qs.values(\'value1\', \'value2\'), which isn\'t valid.'

def _prepare(self):

    if ((self._fields and (len(self._fields) > 1)) or ((not self._fields) and (len(self.model._meta.fields) > 1))):
        raise TypeError(('Cannot    use    a    multi-field    %s    as    a    filter    value.' % self.__class__.__name__))
    return self
