django: assigning foreign key before target model is saved

Question

Assume A and B are django Models, where A has a foreign key field to B:

a = A()
b = B()
a.my_b = b
b.save()
a.save()

Surprisingly a is saved with null in the my_b foreign key field. If you check a.my_b before or after the saves it does refer to the b instance, however it seems that django evaluates the foreign key id itself upon a.my_b = b assignment and not upon a.save(). In complex systems and flows it may be hard to change the order of commands, so simply saving b before assigning it to a could be impractical...

Adding the incredible statement a.my_b=a.my_b between the save commands works, but seems to me a horrid workaround.

Is there a way to configure django to re-evaluate foreign keys upon saving model instances?

Is there a different approach that might solve this issue?

Answer 1

There's a Django ticket that describes this issue, #8892.

In the meantime, there's no configuration setting that changes the behaviour. You'll have to change the order of your save and assignment operations, or do the extra assignment.