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Using Case..When in Django

·2 mins·

I was working on a multi-tenant project and encountered a bug when using Django’s GenericForeignKey with django-tenants. It was using the public schema’s contenttype_id instead of the tenant schema’s contenttype_id.

So, if I have a model of Comment, my django_content_type table would have something like

public.django_content_type

id app_label model
15 comments comment

tenant.django_content_type

id app_label model
19 comments comment

There shouldn’t be a problem here since django-tenants should handle this because it chooses the id of the tenant first and then only use the public as a fall back value. But for some reason, it was sometimes using the public id so comments aren’t appearing at all!

In order to fix this, I opted to remove django_content_type table from all of my tenants and should only use the public’s django_content_type values.

What I had to do was to update the contents inside my models that were using GenericForeignKeys, which in my case is the comments table.

Here’s the model:

# comments/models.py

class Comment(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
    parent = models.ForeignKey("self", null=True, blank=True, on_delete=models.SET_NULL)
    path = models.CharField(max_length=350)
    text = models.TextField()
    timestamp = models.DateTimeField(auto_now_add=True)
    updated = models.DateTimeField(auto_now=True)
    active = models.BooleanField(default=True)
    flagged = models.BooleanField(default=False)

    target_content_type = models.ForeignKey(ContentType, null=True, blank=True, on_delete=models.SET_NULL)
    target_object_id = models.PositiveIntegerField(null=True, blank=True)
    target_object = GenericForeignKey("target_content_type", "target_object_id")

I need to update the target_content_type so that it uses the public id which is 15 instead of 19.

What I needed to do was:

  1. Determine which target ids need to be updated. For example, a comment can be in an Announcement or in a Post. So, we’d have to determine the content_type_id for Announcement and Post in the tenant’s schema and update its value so it uses the one in public.
  2. Update the values using Case..When.
  3. Drop the tenant.django_content_type table so it would always use public.django_content_type.

For number 1, I had to do a GROUP BY to determine which id’s I need to update then get its equivalent in the public schema.

For number 2, I had to use the Case..When syntax. So, for example when the target_content_type_id is 19, then update its value to 15. when the target_content_type_id is 20, then update its value to 12.

I think I need not explain number 3 since it only drops the table.

I created a management command for this so it can be easily executed in production. Here’s the code:

The code could still be improved but it did the job for me.