id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
36064	save() behavior for new model instance with default primary key does not apply to CompositePrimaryKey	Jacob Walls	Csirmaz Bendegúz	"Based on the [https://docs.djangoproject.com/en/dev/ref/models/instances/#how-django-knows-to-update-vs-insert description] of how `save()` is different than `update_or_create()` if the model's primary key has a default, i.e. that only an INSERT is attempted if the pk is not set by the user, I would expect models with a CompositePrimaryKey to behave as if they ""have a default"" if all of their composed fields have a default, or else for this to be called out in the doc.

This test shows this is not the case. The first test showing 2 queries instead of 1 is not such a serious problem, but the second test not raising IntegrityError seems like it could bite a user who was expecting to catch IntegrityError and do a custom update (based on how other models behave since Django 3.0 and #29260).

{{{#!diff
diff --git a/tests/composite_pk/models/tenant.py b/tests/composite_pk/models/tenant.py
index ac0b3d9715..2273908031 100644
--- a/tests/composite_pk/models/tenant.py
+++ b/tests/composite_pk/models/tenant.py
@@ -1,3 +1,4 @@
+import uuid
 from django.db import models
 
 
@@ -46,5 +47,5 @@ class Comment(models.Model):
 
 class Post(models.Model):
     pk = models.CompositePrimaryKey(""tenant_id"", ""id"")
-    tenant = models.ForeignKey(Tenant, on_delete=models.CASCADE)
-    id = models.UUIDField()
+    tenant = models.ForeignKey(Tenant, on_delete=models.CASCADE, default=1)
+    id = models.UUIDField(default=uuid.uuid4)
diff --git a/tests/composite_pk/test_create.py b/tests/composite_pk/test_create.py
index 7c9925b946..af7ceb7df2 100644
--- a/tests/composite_pk/test_create.py
+++ b/tests/composite_pk/test_create.py
@@ -1,6 +1,7 @@
+from django.db import IntegrityError
 from django.test import TestCase
 
-from .models import Tenant, User
+from .models import Post, Tenant, User
 
 
 class CompositePKCreateTests(TestCase):
@@ -14,6 +15,16 @@ class CompositePKCreateTests(TestCase):
             id=1,
             email=""user0001@example.com"",
         )
+        cls.post = Post.objects.create()
+
+    def test_save_default_pk_not_set(self):
+        with self.assertNumQueries(1):
+            Post().save()
+
+    def test_save_default_pk_set(self):
+        with self.assertRaises(IntegrityError):
+            Post(tenant_id=1, id=self.post.id).save()
+
 
     def test_create_user(self):
         test_cases = (
}}}"	Bug	closed	Database layer (models, ORM)	dev	Release blocker	fixed		Csirmaz Bendegúz	Ready for checkin	1	0	0	0	0	0