Django：强制一个字段对于具有相同外键的所有模型对象是唯一的

Question

Suppose I have the following Models defined in django (not tested):

class CarMaker(models.Model):
   name = models.CharField("Name of car maker",
                            max_length=40)


class Car(models.Model):
   car_id = models.IntegerField("ID for this particular car")
   maker = models.ForeignKey("Maker of this car")

Is there a standard django way to ensure that all Car s with the same maker have a unique car_id , without making car_id unique for all Car s?

Eg There are two car makers, "Skoda" and "Renault". There are 400 Car s made by Skoda, and 300 Car s made by Renault. I want to ensure that car_id is unique for all Skodas, and unique for all Renaults, but not necessarily unique for all Car s.

Thanks

Answer 1

You can use the model options unique_together to create this type of constraint. See the Django docs: http://docs.djangoproject.com/en/1.2/ref/models/options/#unique-together

class Car(models.Model):
    car_id = models.IntegerField("ID for this particular car")
    maker = models.ForeignKey("Maker of this car")

    class Meta(object):
        unique_together = ("car_id", "maker")