Django queryset on related field with multiple constraints

ghz 12hours ago ⋅ 1 views

Suppose I have the following models:

class User(models.Model):
    # ... some fields

class Tag(models.Model):
    # ... some fields

class UserTag(models.Model):
    user = models.ForeignKey(User, related_name='tags')
    tag = models.ForeignKey(Tag, related_name='users')
    date_removed = models.DateTimeField(null=True, blank=True)

Now I lets say I want to get all the users that have a given tag that has not yet been removed (ie date_removed=None). If I didn't have to worry about the date_removed constraint, I could do:

User.objects.filter(tags__tag=given_tag)

But I want to get all users who have that given tag and have the tag without a date_removed on it. Is there an easy way in Django to get that in a single queryset? And assume I have millions of Users, so getting any sort of list of User IDs and keeping it in memory is not practical.

Answer

To filter the User model by a Tag that has not been removed (i.e., date_removed is None), you can still achieve this with Django's ORM, but you need to account for the date_removed condition on the UserTag model. Here's how you can do it:

You can use Django's filter and exclude mechanisms to apply the date_removed condition while filtering the User model based on the related Tag.

Solution:

User.objects.filter(tags__tag=given_tag, tags__date_removed__isnull=True)

Explanation:

  • tags__tag=given_tag: This filters the User objects where the related UserTag has the given Tag (this is equivalent to your initial condition).
  • tags__date_removed__isnull=True: This adds a condition that ensures the related UserTag has a date_removed of None (i.e., the tag has not been removed).

Why this works:

  • tags__tag is referring to the related Tag via the UserTag model.
  • tags__date_removed__isnull=True ensures that the UserTag has not been removed (date_removed is None).

Performance Considerations:

Since this is using Django's filter with a ForeignKey relationship (tags is the related name for the UserTag model), this will generate an optimized SQL query that performs the filtering on the database side. No intermediate list of User IDs is kept in memory, and you don't have to manually manage any IDs or memory constraints.

This solution should work efficiently even for a large number of users, since the filtering is handled at the database level.

#py