Django model - Foreign Key as Primary Key

Question

I have the following 2 tables

In models.py

class Foo(models.Model):

    uuid = models.CharField(_('UUID'), primary_key=True, default=uuid4)

and

class FooExt(models.Model):

    uuid = models.ForeignKey(Foo, verbose_name=_('UUID'), primary_key=True)
    time = models.DateTimeField(_('Create DateTime'), auto_now_add=True) 

Basically, I have Foo and FooExt. I want a one-to-one relation between FooExt. That's why I set FooExt's primary key to be foreign key into Foo (not sure if this is the right thing to do).

Now I add an entry into Foo. Does an entry for FooExt automatically get created? Or do I need to manually add an entry to both Foo and FooExt?

Is there anything I can do to get the "automatic" add feature? Conceptually, these 2 tables describe the same thing, but I just don't want to pollute Foo with extra information. So it'd be great if an add to Foo automatically creates a corresponding FooExt.

Answer 1

1. If you want an OneToOne relation, then use models.OneToOneField instead of models.ForeignKey. with foreign keys you will need add unique=True in you ForeignKey:

class Foo(models.Model):
    uuid = models.CharField(_('UUID'), primary_key=True, default=uuid4)

class FooExt(models.Model):
    uuid = models.OneToOneField(Foo, verbose_name=_('UUID'), primary_key=True)
    time = models.DateTimeField(_('Create DateTime'), auto_now_add=True)

2. No, an entry for FooExt don't get created when you create a Foo instance, you need to manually add an entry to both Foo and FooExt. think in Places and Restaurants, many places can be restaurants, but no all the places are restaurants.

3. if you like an automatic add feature inside Foo that create a FooExt instance, then you can overload the save method inside Foo that create and save FooExt instance too, something like this:

class Foo(models.Model):
....
....
     def save(self, *args, **kwargs):
        super(Foo, self).save(*args, **kwargs)
        foo_ext = FooExt()
        foo_ext.uuid = self
        foo_ext.save()