@classmethod
def iter_unique_field_combinations(cls, model_cls):
    """ Returns an iteratorable of unique constraints (set of
        field names) on the model.
        
        .. note::
            Relation considered to be unique if the set of columns used as
            foreign keys (can be multiple in case of composite key) has a
            unique constraint.
        
            *Relations can be unique together with other fields.*
        
        :param model_cls: ORM model class for which relations are going to be
            iterated.
        :returns: Returns an iteratorable of unique constraints.
        """
    raise NotImplementedError()
