How can I make a Model field value unique (unique=True) but only comparing it to other models from that User?

Question

I have a Tag model, that must have a name field that needs to be unique, but only for that user's tags.

So that one user can't create two 'tagname' tags, but many different users can (create a tag with the same name).

The field must be unique inside the models that relate to the User.

This is the Model.

class User(AbstractUser): email = models.EmailField(unique=True) class Tag(models.Model): name = models.CharField(max_length=30, unique=True) user = models.ForeignKey(User, null=True, on_delete=models.CASCADE, related_name='tags')

This looks like an answer to a similar question: stackoverflow.com/a/2201687/129655 — Platinum Azure
– Platinum Azure, Commented Jul 14, 2019 at 23:32
It does. The Meta tag unique_together is what's needed: docs.djangoproject.com/en/2.2/ref/models/options — Deepstop
– Deepstop, Commented Jul 14, 2019 at 23:37

Community · Accepted Answer · 2020-06-20 09:12:55Z

If you are using Django 2.1 or older you can use unique_together like this:

class Tag(models.Model): name = models.CharField(max_length=30, unique=True) user = models.ForeignKey(User, null=True, on_delete=models.CASCADE, related_name='tags') class Meta: unique_together = ['name', 'user']

If you are using Django 2.2 you can use UniqueConstraint instead of unique_together like this:

class Tag(models.Model): name = models.CharField(max_length=30, unique=True) user = models.ForeignKey(User, null=True, on_delete=models.CASCADE, related_name='tags') class Meta: constraints = [ models.UniqueConstraint(fields=['name', 'user'], name='give_it_some_name') ]

As the docs state:

Use UniqueConstraint with the constraints option instead.

UniqueConstraint provides more functionality than unique_together. unique_together may be deprecated in the future.

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