Updating a model instance with a composite primary key through .save() results in unnecessary re-assignment of primary key fields

[Simon Charette]

Looking at the code that gets involved and the generated SQL It appears that every save that results in an UPDATE for a model with a composite primary key also includes all the primary key field in the updated fields.

For example, for a model of the form

class User(models.Model):
    tenant = models.ForeignKey(Tenant)
    id = models.UUIDField(default=uuid.uuid4)
    username = models.EmailField()

Then doing

user = User.objects.create(tenant=tenant, id=1, email="foo@bar.org")
user.email = "something@example.com"
user.save()
# Will result in UPDATE user SET tenant_id = %s, id = %s, email = %s WHERE tenant_id = %s AND id = %s
#                                ^^^^^^^^^^^^^^^^^^^^^^^  <- tenant_id and id should not be marked for UPDATE

Will result in the following SQL

INSERT INTO user(tenant_id, id, email) VALUES (%s, %s, %s)
UPDATE user SET tenant_id = %s, id = %s, email = %s WHERE tenant_id = %s AND id = %s

But the UPDATE statement should not set tenant_id and id as they are part of the primary key lookup. The UPDATE should be

UPDATE user SET email = %s WHERE tenant_id = %s AND id = %s

which is due to a non-audited usage of .primary_key in `save_tables

django/db/models/base.py

@@ -1091 +1091 @@
         for a single table.
         """
         meta = cls._meta
+        pk_fields = meta.pk_fields
         non_pks_non_generated = [
             f
             for f in meta.local_concrete_fields
-            if not f.primary_key and not f.generated
+            if f not in pk_fields and not f.generated
         ]
 
         if update_fields:

and makes me fear there might be others lying around that went unnoticed. If you grep for .primary_key you'll notice that most of the checks against this field need to be adjusted to use field in opts.pk_fields instead which makes me wonder if we took the right decision by not setting this flag to True when included as a member of CompositePrimaryKey.

[Sarah Boyce <42296566+sarahboyce@…>]

In af6336f:

Fixed #36074 -- Excluded composite primary key fields on save() updates.