Django Models how to add unique constraint

Question

Unsimilar to for example this case I am trying to allow only one entry in a database for one user:

class Station(models.Model):
    serial = models.CharField("serial", max_length = 31, unique = True)
    user = models.ForeignKey(User, on_delete = models.CASCADE, )

What I want is that one user can only have zero or one Station (serial).

score 1 · Accepted Answer · answered Dec 23 '20 at 09:35

1

This can be achieved using a one-to-one relationship. This means a user can have zero or exactly one station.

class Station(models.Model):
    serial = models.CharField("serial", max_length = 31, unique = True)
    user = models.OneToOneField(
        User,
        on_delete=models.CASCADE,
    )

answered Dec 23 '20 at 09:35

Nepo Znat

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can I edit the message a user sees when trying to add a second serial? – xtlc Dec 23 '20 at 09:40
I think this depends on your use case. Are you using model forms, Django Rest framework, or something else? – Nepo Znat Dec 23 '20 at 09:43
I was more thinking about the admin tbh. – xtlc Dec 23 '20 at 10:01

score 1 · Answer 2 · answered Dec 23 '20 at 09:36

You can set unique=true on your ForeignKey, but you do have wrong relationship type as you should have One-to-one

A one-to-one relationship. Conceptually, this is similar to a ForeignKey with unique=True, but the “reverse” side of the relation will directly return a single object.

class Station(models.Model):
    serial = models.CharField("serial", max_length=31, unique=True)
    user = models.OneToOneField(
        User,
        on_delete=models.CASCADE,
        null=True,
        blank=True
    )

Django Models how to add unique constraint

2 Answers2