Why would someone set primary_key=True on an One to one reationship (OneToOneField)?

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I was watching a video on django-orm and the instructor stated that:

We should set primary_key=True to prevent a Model from having duplicate rows in a OneToOne relationship (Ex: Prevent a user from having multiple profiles).

I know this statement is wrong! AFAIK, an OneToOne field is just a ForeignKey with unique parameter set to True. But I got curious and looked-up the Django documentation, Sure enough they are using primary=True in their example.

class Place(models.Model):
    name = models.CharField(max_length=50)
    address = models.CharField(max_length=80)

class Restaurant(models.Model):
    place = models.OneToOneField(
        Place,
        on_delete=models.CASCADE,
        primary_key=True,
    )

So, why would someone set primary_key=True on an OneToOne relation? My guess is that it's just reasonable to have that field as a primary key and there is no technical background behind it.

database-design django-orm django
1 Answers

This is a pattern to implement object-oriented inheritance in a relational database, for example as this article of Oracle discusses.

Indeed, it means that one can define a Place, and for that Place create a Restaurant model as well. It has a OneToOneField(…) to the "parent" model. The OneToOneField prevents that one can define two (or more) Restaurants for the same Place.

Usually it is defined as a primary key, since then it shares the same "primary code space", and it removes a column that is otherwise used to do the mapping and thus would make the database larger.

Django will implement this the same way. If we define this as:

class Place(models.Model):
    name = models.CharField(max_length=50)
    address = models.CharField(max_length=80)

class Restaurant(Place):
    pass

then it will be implemented as:

mysql> describe place;
+---------+-------------+------+-----+---------+----------------+
| Field   | Type        | Null | Key | Default | Extra          |
+---------+-------------+------+-----+---------+----------------+
| id      | int         | NO   | PRI | NULL    | auto_increment |
| name    | varchar(50) | NO   |     | NULL    |                |
| address | varchar(80) | NO   |     | NULL    |                |
+---------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> describe restaurant;
+--------------+------+------+-----+---------+-------+
| Field        | Type | Null | Key | Default | Extra |
+--------------+------+------+-----+---------+-------+
| place_ptr_id | int  | NO   | PRI | NULL    |       |
+--------------+------+------+-----+---------+-------+

It thus will add a primary key named place_ptr_id that refers to the place table. This originates from a OneToOneField that Django adds to the Restaurant model named place_ptr.

We should set primary_key=True to prevent a Model from having duplicate rows in a OneToOne relationship (Ex: Prevent a user from having multiple profiles)

This makes no sense, since a OneToOneField is in essence a ForeignKey with unique=True [Django-doc]. So this is already enforced by the OneToOneField, it is not necessary to make it a primary key.

Something that can not be done with the above modeling is preventing that an Place is a Restaurant and Library at the same time.

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