Why can't uniqueness be enforced on Django ManyToMany field?

Question

I'm creating a simple binary rating system for my website, it is a study materials website with summaries that can be rated +1 or -1.

I've defined to ManyToMany fields between my Summary model and User model like so:

users_rated_positive = models.ManyToManyField(
    User, blank=True, related_name='summaries_rated_positive')
users_rated_negative = models.ManyToManyField(
    User, blank=True, related_name='summaries_rated_negative')

However it is obvious that a given user can not rate a given summary more than once.

I tried setting unique=True but Django doesn't seem to let me do that.

Is there are more optimal way to store the ratings that aligns better with my intentions?

Did you check that you can effectively add duplicate relations ? This answer seems to imply that duplicate relations are not permitted: http://stackoverflow.com/questions/4870935/django-enforcing-manytomanyfield-unique-items — Dric512, Aug 25 '15 at 09:10
I never tried it myself so cannot be sure, but this is what is described in the answer I linked. You may try to reproduce the test case that is in the answer to check. — Dric512, Aug 25 '15 at 09:17

score 4 · Answer 1 · edited Jun 20 '20 at 09:12

Consider using a ManyToMany through table.

class Summary(models.Model):
    user_ratings = models.ManyToManyField(
        'User', blank=True, through='UserRating',
    )


class UserRating(models.Model):
    user = models.ForeignKey('User')
    summary = models.ForeignKey('Summary')
    is_positive = models.BooleanField()

    class Meta:
        unique_together = ('user', 'summary')

Then you can query users who reviewed the summary positively with something like

summary.user_ratings.filter(is_positive=True)
summary.user_ratings.filter(is_positive=False)

The unique_together property ensures that a user can't review the same summary twice.

You could even retain similar syntax using a custom queryset. Here's a guess at what that might look like:

class SummaryQuerySet(models.QuerySet):

    def users_rated_negative(self):
        return self.user_ratings.filter(is_positive=False)

    def users_rated_positive(self):
        return self.user_ratings.filter(is_positive=True)


class Summary(models.Model):
    user_ratings = models.ManyToManyField(
        'User', blank=True, through='UserRating',
    )

    objects = SummaryQuerySet.as_manager()

Now the in-use syntax is summary.users_rated_positive

A similar custom queryset for User would give you the summaries_rated_positive property, if you needed that too.

Why can't uniqueness be enforced on Django ManyToMany field?

1 Answers1