How to prevent two ForeignKey entries in a model from being the same for each record in Django

Question

I have two foreign key fields that point to the same model. I would like to prevent them from having the same value. Here is my code that does not work

class Match(models.Model):
    team_a = models.ForeignKey(Team, related_name='team_a', on_delete=models.CASCADE)
    team_b = models.ForeignKey(Team, related_name='team_b', on_delete=models.CASCADE)
    match_date_time = models.DateTimeField(validators=[validate_matchdate])
    winner = models.CharField(choices=CHOICES, max_length=50, blank=True, null=True)
    
    
    def clean(self):
        direct = Match.objects.filter(team_a = self.team_a, team_b=self.team_b)
        reverse = Match.objects.filter(team_a = self.team_b, team_b=self.team_a)
        
        if direct.exists() & reverse.exists():
            raise ValidationError('A team cannot be against itself')

Maybe you need to use `and` instead of `&` in your if statement. — Moein Babapour, Nov 28 '22 at 01:24

score 1 · Accepted Answer · answered Nov 28 '22 at 21:46

1

You can add a CheckConstraint ^[Django-doc] such that the database will enforce this:

from django.db.models import F, Q


class Match(models.Model):
    team_a = models.ForeignKey(
        Team, related_name='matches_as_a', on_delete=models.CASCADE
    )
    team_b = models.ForeignKey(
        Team, related_name='matches_as_b', on_delete=models.CASCADE
    )
    match_date_time = models.DateTimeField(validators=[validate_matchdate])
    winner = models.CharField(
        choices=CHOICES, max_length=50, blank=True, null=True
    )

    class Meta:
        constraints = [
            models.CheckConstraint(
                check=~Q(team_a=F('team_b')), name='not_play_against_itself'
            )
        ]

answered Nov 28 '22 at 21:46

Willem Van Onsem

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It did not prevent the duplication yet – sajeyks mwangi Nov 28 '22 at 22:04
@sajeyksmwangi: this is not duplication, this prevents that `team_a` and `team_b` will be the same for *any* record. – Willem Van Onsem Nov 28 '22 at 22:11
Sorry for the misuse of the term, I meant to say that even with the constraint, Django admin is still allowing `team_a` and `team_b` to be the same on any given record. – sajeyks mwangi Nov 28 '22 at 22:14
@sajeyksmwangi: I made a typo, it is with `~`. You also need to migrate the database. – Willem Van Onsem Nov 28 '22 at 22:28
I have removed the `~` and migrated but nothing. – sajeyks mwangi Nov 28 '22 at 22:36
@sajeyksmwangi: no, it should be *with* the `~`. – Willem Van Onsem Nov 28 '22 at 22:55
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/249973/discussion-between-sajeyks-mwangi-and-willem-van-onsem). – sajeyks mwangi Nov 28 '22 at 22:59
@sajeyksmwangi: what version of Django do you use? – Willem Van Onsem Nov 29 '22 at 13:33
Here: `Django==4.1.3` – sajeyks mwangi Nov 29 '22 at 20:38
@sajeyksmwangi: and it did generate a migration when you did `makemigrations`? – Willem Van Onsem Nov 29 '22 at 20:53
No, it did not. – sajeyks mwangi Nov 29 '22 at 21:21
@sajeyksmwangi: then something is wrong when you defined the constraints. Are you sure the `Meta` is an *inner class* (not a method), and that you defined a list named `constraints`? – Willem Van Onsem Nov 29 '22 at 21:23

How to prevent two ForeignKey entries in a model from being the same for each record in Django

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