class Model1(models.Model):
  ...

class Model2(models.Model):
    model1 = models.ForeignKey(Model1)
    attr = models.IntegerField(default=1)

>>> m1.model2_set.all()[0].attr += 1
>>> m1.model2_set.all()[0].attr
1
>>> m2 = m1.model2_set.all()[0]
>>> m2.attr += 1
2

>>> m2.model2_set.all()[0] == m1.model2_set.all()[0] # should be true

>>> m1.model2_set.all()[0].attr += 1
>>> m1.model2_set.all()[0].attr

>>> m2 = m1.model2_set.all()[0]
>>> m2.attr += 1
>>> m2.attr
2
>>> m2 = m1.model2_set.all()[0]
>>> m2.attr
1