'A m2m can be followed through another m2m.'

def test_m2m_then_m2m(self):

    authors = list(Author.objects.all())
    with self.assertNumQueries(2):
        prefetch_related_objects(authors, 'books__read_by')
    with self.assertNumQueries(0):
        self.assertEqual([[[str(r) for r in b.read_by.all()] for b in a.books.all()] for a in authors], [[['Amy'], ['Belinda']], [['Amy']], [['Amy'], []], [['Amy', 'Belinda']]])
