A OneToOneField is used when you want to create a One-to-one relations. In this article, we will go over the usecase for such a field and how it should be implemented.

When to use a one-to-one relationship? #

You generally won’t use one-to-one relationships that often and I also won’t really advise them, except for very specific cases, like this one:

Say you have a user model. In that model you also store details about their profile (bio, address, profile picture etc). When that table gets very big, a lot of columns could influence the speed of retrieving items from that table. In that case, it might be a good idea to split the user details from the profile details and create a separated table from that. Connecting the two through a OneToOneField would make sense here, since generally a user won’t have two profiles or vice versa.

OneToOneField example #

Let’s take the previous example and turn that into some code. It would look like this:

class User(AbstractBaseUser):
    profile = OneToOneField(Profile, on_delete=models.CASCADE)
    # other fields you want to add beside the default ones from AbstractBaseUser

class Profile(models.Model):
    bio = TextField()
    # more profile fields

You can then get the bio from the user by doing this:

u = User.objects.get(id=1)

Another advantage of this approach is that you can generally cache the Profile table for a longer time. But, again, I generally wouldn’t recommend this approach as it makes things a lot more complicated.