def METHOD_NAME(self):
    table1 = Table([], 'A', 'B')
    table2 = Table([], 'C', 'D')
    link1 = DataLink(table1, table2)
    DataLink(table1, table2)
    self.assertEqual(len(Link.registry[table1]), 1)
    self.assertIn(link1, Link.registry[table1])