Django 3 - Model.save() when providing a default for the primary key

Question

I'm working on upgrading my project from Django 2 to Django 3, I have read their release notes of Django 3 and there is a point that I don't really understand what it will impact to my current project. Here they say:

As I understand, if we try to call Model.save(), it would always create a new record instead of updating if the model is an existing record. For example:

car = Car.objects.first()
car.name = 'Honda'
car.save() # does it INSERT or UPDATE? I suspect it is an "INSERT" statement as their explanation and "UPDATE" statement in Django 2.

I have had an experimented and it is still the same behaviour as Django 2, not sure what they mean.

In [5]: u = User.objects.first()                                                                                                                                                                          
(0.001) SELECT "accounts_user"."id", "accounts_user"."password", "accounts_user"."last_login", "accounts_user"."is_superuser", "accounts_user"."username", "accounts_user"."first_name", "accounts_user"."last_name", "accounts_user"."is_staff", "accounts_user"."is_active", "accounts_user"."date_joined", "accounts_user"."email", "accounts_user"."avatar", "accounts_user"."last_location"::bytea, "accounts_user"."uuid", "accounts_user"."country", "accounts_user"."city", "accounts_user"."phone" FROM "accounts_user" ORDER BY "accounts_user"."id" ASC LIMIT 1; args=()

In [6]: u.save()                                                                                                                                                                                          
(0.006) UPDATE "accounts_user" SET "password" = 'pbkdf2_sha256_sha512$180000$FbFcNuPMrOZ6$GwIftEo+7+OpsORwn99lycye46aJn/aJNAtc50N478Y=', "last_login" = NULL, "is_superuser" = false, "username" = 'email0@mail.com', "first_name" = 'Noah', "last_name" = 'Spencer', "is_staff" = false, "is_active" = true, "date_joined" = '2020-05-12T07:06:20.605650+00:00'::timestamptz, "email" = 'email0@mail.com', "avatar" = 'account/user_avatar/example_HseJquC.jpg', "last_location" = NULL, "uuid" = 'f6992866-e476-409e-9f1b-098afadce5b7'::uuid, "country" = NULL, "city" = NULL, "phone" = NULL WHERE "accounts_user"."id" = 1; args=('pbkdf2_sha256_sha512$180000$FbFcNuPMrOZ6$GwIftEo+7+OpsORwn99lycye46aJn/aJNAtc50N478Y=', False, 'email0@mail.com', 'Noah', 'Spencer', False, True, datetime.datetime(2020, 5, 12, 7, 6, 20, 605650, tzinfo=<UTC>), 'email0@mail.com', 'account/user_avatar/example_HseJquC.jpg', UUID('f6992866-e476-409e-9f1b-098afadce5b7'), 1)

Update:

In [38]: u1 = User.objects.first()                                                                                                                                                                        
(0.000) SELECT "accounts_user"."id", "accounts_user"."password", "accounts_user"."last_login", "accounts_user"."is_superuser", "accounts_user"."username", "accounts_user"."first_name", "accounts_user"."last_name", "accounts_user"."is_staff", "accounts_user"."is_active", "accounts_user"."date_joined", "accounts_user"."email", "accounts_user"."avatar", "accounts_user"."last_location"::bytea, "accounts_user"."uuid", "accounts_user"."country", "accounts_user"."city", "accounts_user"."phone" FROM "accounts_user" ORDER BY "accounts_user"."id" ASC LIMIT 1; args=()

In [39]: u1.pk                                                                                                                                                                                            
Out[39]: 1

In [40]: u2 = User(pk=1)                                                                                                                                                                                  

In [41]: u2.email = 'email@email.com'                                                                                                                                                                     

In [42]: u2.save()                                                                                                                                                                                        
(0.006) UPDATE "accounts_user" SET "password" = '', "last_login" = NULL, "is_superuser" = false, "username" = 'email@email.com', "first_name" = '', "last_name" = '', "is_staff" = false, "is_active" = true, "date_joined" = '2020-05-13T01:20:47.718449+00:00'::timestamptz, "email" = 'email@email.com', "avatar" = '', "last_location" = NULL, "uuid" = '89ba0924-03a7-44d2-bc6d-5fd2dcb0de0b'::uuid, "country" = NULL, "city" = NULL, "phone" = NULL WHERE "accounts_user"."id" = 1; args=('', False, 'email@email.com', '', '', False, True, datetime.datetime(2020, 5, 13, 1, 20, 47, 718449, tzinfo=<UTC>), 'email@email.com', '', UUID('89ba0924-03a7-44d2-bc6d-5fd2dcb0de0b'), 1)

Rather than posting images quote text from docs – iklinac May 12 '20 at 15:09 — iklinac, May 12 '20 at 15:09

JPG · Accepted Answer · 2020-05-15T03:09:39.257

Consider this example. Suppose we have a simple model as

CONSTANT = 10


def foo_pk_default():
    return CONSTANT


class Foo(models.Model):
    id = models.IntegerField(primary_key=True, default=foo_pk_default)
    name = models.CharField(max_length=10)

The main thing I have done in this example is, I did set a default callable function for Primary Keys. Also, I returned only a single value from the function, for the sake of demonstration.

## Django 2.2
In [5]: foo_instance_1 = Foo(name='foo_name_1')

In [6]: foo_instance_1.save()

In [7]: print(foo_instance_1.__dict__)
{'_state': , 'id': 10, 'name': 'foo_name_1'}

In [8]: foo_instance_2 = Foo(name='foo_name_2')

In [9]: foo_instance_2.save()

In [10]: print(foo_instance_2.__dict__)
{'_state': , 'id': 10, 'name': 'foo_name_2'}

## Django 3.X
In [6]: foo_instance_1 = Foo(name='foo_name_1')

In [7]: foo_instance_1.save()

In [8]: print(foo_instance_1.__dict__)
{'_state': , 'id': 10, 'name': 'foo_name_1'}

In [9]: foo_instance_2 = Foo(name='foo_name_2')

In [10]: foo_instance_2.save()
# Raised "IntegrityError: UNIQUE constraint failed: music_foo.id"

Conclusion

In Django<3.0, the Model.save() will do an update or insert operation if there is a PK value associated with the model instance whereas in Django>=3.0, only perform an insert operation hence the UNIQUE constraint failed exception.

Impact of this change in the current project

Since this Django change is only applicable when a new instance is created and we usually don't set any default value functions for Primary Keys.

In short, this change will not make any problem unless you are providing default value during model instance creation.

This is a perfect answer. Thanks! I will award the bounty once it is available (it says there are 17 hours left). — Thanh Nguyen, May 15 '20 at 08:52
Any idea _why_ the Django project has made this change? It breaks quite a lot of code for me. — bgrant, Mar 28 '22 at 22:12

score 0 · Answer 2 · answered May 12 '20 at 15:09

0

It should be read carefully

no longer attempts to find a row when saving a new Model instance and a default value for the primary key is provided,

So in case you are going to create new object with id that is already in database it would now fail instead of having behavior similar to update_or_create as it will now preform INSERT statement instead of UPDATE

answered May 12 '20 at 15:09

iklinac

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It seems not like that, it still performs an UPDATE statement. Please see my update in the question post for the example. – Thanh Nguyen May 13 '20 at 01:24

Django 3 - Model.save() when providing a default for the primary key

2 Answers2

Conclusion

Impact of this change in the current project