Django - Multiple optional ForeignKey

class Truc(models.Model):
  name = models.CharField(max_length=50, unique=True)
  machin1 = models.ForeignKey(Machin1, blank=True, null=True)
  machin2 = models.ForeignKey(Machin2, blank=True, null=True)
  machin3 = 开发者_Python百科models.ForeignKey(Machin3, blank=True, null=True)

I would like Truc to be linked to one machin, either machin1 either machin2 or machin3.

Only one link, so this kind of validation:

machin1 xor machin2 xor machin3

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If it is only ever linked to one use a generic relation:

class Truc(models.Model):
    name = models.CharField(max_length=50, unique=True)
    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    machin_object = generic.GenericForeignKey('content_type', 'object_id')