[Fixed]-How to ensure that two foreign keys both belong to the same user

Try Django Smart Selects we use them in our project to solve that exact problem

You can try to find a workaround with unique_together (https://docs.djangoproject.com/en/dev/ref/models/options/#unique-together) by overriding it for your special needs.

But it might not be easy: Django Unique Together (with foreign keys)

But it’s never wrong to check values before insert

Maybe teacher should be a separate class, and teacher in Task class should be related with teacher from Student class. Maybe usage the through argument is option for it.