def key(self):
        lst = [getattr(self, k) for k in self.keylist]
        return len(lst) == 1 and lst[0] or tuple(lst)
test that create a supplement issue, with ``number``, ``suppl_text`` and empty ``volume`` fields.
dataset/ETHPy150Open perseas/Pyrseas/pyrseas/dbobject/__init__.py/DbObject.key--crossover with--dataset/ETHPy150Open scieloorg/scielo-manager/scielomanager/api/tests_resources_v2.py/IssuesRestAPITest.test_suppl_number_filter_without_volume