Unique together between field and model connected by ForeignKey django

Question

I have two models.

class Post(models.Model):
    title = models.CharField(max_length=150)

class Attachment(models.Model):
    type = models.CharField(choices=AttachmentChoices.get_choices(), max_length=15)
    link = models.URLField()
    post = models.ForeignKey('Post', on_delete=models.PROTECT, related_name='attachments')

I want to do something like this in meta class of Post model

    class Meta:
        unique_together = [('title', 'attachments')]

but it dosn't work couse django can't find attachments field. Is there a solution provided by django that can solve this problem.

You should use OneToOne field for this use-cases: https://docs.djangoproject.com/en/4.1/topics/db/examples/one_to_one/ — Marco, Mar 11 '23 at 00:14
in my case there shoud be more then one attachment for every post — user15125625, Mar 11 '23 at 09:00

score 1 · Accepted Answer · answered Mar 11 '23 at 13:11

You can use UniqueConstraint and also try to create a many-to-many relationship between Post and Attachment models using attachments field, try the following:

from django.db.models import UniqueConstraint

class Post(models.Model):
    title = models.CharField(max_length=150)
    attachments = models.ManyToManyField('Attachment', related_name='posts')

    class Meta:
        constraints = [
            UniqueConstraint(fields=['title'], name='unique_title_with_attachments')
        ]

Now, django will create a unique constraint on the combination of title and attachments fields. This means that when you create a new Post instance, Django will check if there is already a Post instance with the same title and attachments combination, and if so, it will raise a ValidationError.

Unique together between field and model connected by ForeignKey django

1 Answers1