Fixed #29260 -- Skipped an UPDATE when adding a model instance with primary key that has a default.

django/db/models/base.py

@@ -15,6 +15,7 @@ from django.db import (
     DEFAULT_DB_ALIAS, DJANGO_VERSION_PICKLE_KEY, DatabaseError, connection,
     connections, router, transaction,
 )
+from django.db.models import NOT_PROVIDED
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.constraints import CheckConstraint, UniqueConstraint
 from django.db.models.deletion import CASCADE, Collector
@@ -842,6 +843,14 @@ class Model(metaclass=ModelBase):
         if not pk_set and (force_update or update_fields):
             raise ValueError("Cannot force an update in save() with no primary key.")
         updated = False
+        # Skip an UPDATE when adding an instance and primary key has a default.
+        if (
+            not force_insert and
+            self._state.adding and
+            self._meta.pk.default and
+            self._meta.pk.default is not NOT_PROVIDED
+        ):
+            force_insert = True
         # If possible, try an UPDATE. If that doesn't update anything, do an INSERT.
         if pk_set and not force_insert:
             base_qs = cls._base_manager.using(using)

tests/basic/models.py

@@ -3,6 +3,8 @@ Bare-bones model
 
 This is a basic model with only two non-primary-key fields.
 """
+import uuid
+
 from django.db import models
 
 
@@ -40,3 +42,7 @@ class SelfRef(models.Model):
         # This method intentionally doesn't work for all cases - part
         # of the test for ticket #20278
         return SelfRef.objects.get(selfref=self).pk
+
+
+class PrimaryKeyWithDefault(models.Model):
+    uuid = models.UUIDField(primary_key=True, default=uuid.uuid4)

tests/basic/tests.py

@@ -10,7 +10,10 @@ from django.test import (
 )
 from django.utils.translation import gettext_lazy
 
-from .models import Article, ArticleSelectOnSave, FeaturedArticle, SelfRef
+from .models import (
+    Article, ArticleSelectOnSave, FeaturedArticle, PrimaryKeyWithDefault,
+    SelfRef,
+)
 
 
 class ModelInstanceCreationTests(TestCase):
@@ -130,6 +133,11 @@ class ModelInstanceCreationTests(TestCase):
         # ... but there will often be more efficient ways if that is all you need:
         self.assertTrue(Article.objects.filter(id=a.id).exists())
 
+    def test_save_primary_with_default(self):
+        # An UPDATE attempt is skipped when a primary key has default.
+        with self.assertNumQueries(1):
+            PrimaryKeyWithDefault().save()
+
 
 class ModelTest(TestCase):
     def test_objects_attribute_is_only_available_on_the_class_itself(self):