'A m2m can be followed through another m2m.'

def test_m2m_then_m2m(self):

    with self.assertNumQueries(3):
        qs = Author.objects.prefetch_related('books__read_by')
        lists = [[[str(r) for r in b.read_by.all()] for b in a.books.all()] for a in qs]
        self.assertEqual(lists, [[['Amy'], ['Belinda']], [['Amy']], [['Amy'], []], [['Amy', 'Belinda']]])
