How to restrict a Django model to a single one-to-one relation?

Question

I am currently building a Django app that requires two different user types and I know that storing authentication information across multiple models/tables is a bad idea.

As such, I have created a User model to handle the authentication information (username, password, etc.). I have then created two different models, one for the buyers and one for the sellers, each with their own unique fields and one-to-one relationship to the User model.

Now, I thought this would work, but the problem is that it is still possible for a different buyer and seller to have the same User relation. How can I prevent this and restrict the User model to only a single one-to-one relation?

Answer 1

You can use unique_together to achieve your goal like this:

class Actor(models.Model):
    """An abstract base class for Buyer and Seller"""
    
    # We define an id file in the abstract class
    # this way the id is unique for Buyer and Seller
    id = models.AutoField(primary_key=True)
    
    class Meta:
        abstract = True

class Buyer(Actor):
    user_id = models.ForeignKey(User, ...)
    
    class Meta:
        unique_together = ('id', 'user_id',)

class Seller(Actor):
    user_id = models.ForeignKey(User, ...)
    
    class Meta:
        unique_together = ('id', 'user_id',)

With this example, one Buyer is linked to only one user and one Seller is linked to only one user too.

Answer 2

you can add role in actor's model and make a pre_save signal before save user in buyer or seller model