CODEWITHSUNDEEP
pythondjangopython-3.xdjango-models
Lorenzo
Lorenzo

Reputation: 143

Creating an object with more than 1 foreign key in a Django project with multiple databases causing error

In my Django project, I have 2 databases. Everything works perfectly except for when I try to create a model with 2 foreign keys. The database router locks up and gives me a Cannot assign "<FKObject: fk_object2>": the current database router prevents this relation. even if both foreign keys come from the same database and I've yet to save the object in question. Code below

1 fk_object1 = FKObject.objects.using("database2").get(pk=1)
2 fk_object2 = FKObject.objects.using("database2").get(pk=2)
3
4 object = Object()
5 object.fk_object1 = fk_object1
6 object.fk_object2 2 = fk_object2
7 object.save(using="database2")

The problem arises on line 6, before the object is even saved into the database so I'm assuming that Django somehow calls Object() with database1 even though it hasn't been specified yet.

Does anyone know how to deal with this?

Upvotes: 0

Views: 483

Answers (1)

Lorenzo
Lorenzo

Reputation: 143

So I ended up finding a work around as such:

As it turns out, my suspicions were only partially true. Calling Model() does not cause Django to assume it is to use the default database but setting a foreign key does. This would explain why my code would error out at line 6 and not at line 5 as by this point in time, Django already assumes that you're using the default database and as fk_object2 is called from database2, it errors out for fear of causing an inter-database relation.

To get around this, I used threading.current_thread() as so:

class Command(BaseCommand):
    current_thread().db_name = "database2"

    def handle(self, **args, **kwargs):
        # Do work here


class DatabaseRouter(object):
    db_thread = threading.current_thread()

    def db_for_read(self, model, **hints):
        try:
            print("Using {}".format(db_thread.db_name))
            return db_thread.db_name
        except AttributeError:
            return "default"

    def db_for_write(self, model, **hints):
        try:
            print("Using {}".format(db_thread.db_name))
            return db_thread.db_name
        except AttributeError:
            return "default"

This way, my 2nd database is used every time, thereby avoiding any possible relation inconsistencies.

Upvotes: 2

Related Questions