Support for Double underscore '__' traverse when relationships are between models from two different databases?

Using Django Using the ORM
1 
class A(models.Model): 
    id = models.AutoField(primary_key=True)
    b = models.ForeignKey(B, on_delete=models.CASCADE, db_constraint=False )
class B(models.Model):
    someinfo = model.CharField()

# model A are from db named 'database2'
# model B are from db named 'default'

DB router is correctly set for both models using the app label of each model.
Model A and B are from two different Django apps, and are put together here for simplicity.

Now if I do

A.objects.all().values('b__someinfo')

I wouldn’t get any value for ‘b__someinfo’ as b ‘does not exist in database2’ despite I have correctly set the db router to route to ‘default’ for model B and route to ‘database2’ for model A.

I have read that there are limitations for multi db relations in Django, just wants to know if I’m doing anything wrong here or will this be supported anytime soon.

2

Quoting directly from the docs at Cross-database relations:

Django doesn’t currently provide any support for foreign key or many-to-many relationships spanning multiple databases. If you have used a router to partition models to different databases, any foreign key and many-to-many relationships defined by those models must be internal to a single database.

So no, what you’re trying isn’t going to work.

As far as it ever being supported, :man_shrugging: . I’m sure if someone came up with a way of handling this that didn’t cause more potential problems than it solves, it would be considered.

1 Like