gistfile1.py

class Model1(models.Model):
  field1 = models.CharField()
  field2 = models.BooleanField()
  obj2 = models.OneToOneField('Model2', null=True, blank=True)
  obj3 = models.OneToOneField('Model3', null=True, blank=True)

class Model2(models.Model):
  ...
  @classmethod
  def get_model1(cls, some_unique_model2_field):
    model2 = Model2.objects.get(some_unique_model2_field=some_unique_model2_field)
    model1, created = Model1.get_or_create(model2=model2, defaults={...})
    return model1

class Model3(models.Model):
  ...