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
36073	Custom field without get_db_prep_value() errors on updates if part of composite primary key	Jacob Walls		"A model with a custom field only implementing `get_db_prep_save()` but not `get_db_prep_value()` fails on `save()` when used in a composite primary key, but only for updates. Inserts work fine.

This fell out of #36062 where the test model I adjusted happened to have a [https://github.com/django/django/blob/a9c79b462923ce366101db1b5a3fff3d1dad870c/tests/serializers/models/base.py#L128-L149 custom field with these characteristics.]

I didn't get as far as verifying this to be relevant, but I noticed while stepping through the code that `SQLInsertCompiler` has [https://github.com/django/django/blob/a9c79b462923ce366101db1b5a3fff3d1dad870c/django/db/models/sql/compiler.py#L1814 logic to add `pk` to the list of fields], so I wonder if it's not being prepared the same way in `SQLUpdateCompiler`?

{{{#!diff
diff --git a/tests/custom_pk/fields.py b/tests/custom_pk/fields.py
index 2d70c6b6dc..135b315ffe 100644
--- a/tests/custom_pk/fields.py
+++ b/tests/custom_pk/fields.py
@@ -59,6 +59,27 @@ class MyWrapperField(models.CharField):
         return value
 
 
+class MyWrapperFieldWithoutGetDbPrepValue(models.CharField):
+    def to_python(self, value):
+        if not value:
+            return
+        if not isinstance(value, MyWrapper):
+            value = MyWrapper(value)
+        return value
+
+    def from_db_value(self, value, expression, connection):
+        if not value:
+            return
+        return MyWrapper(value)
+
+    def get_db_prep_save(self, value, connection):
+        if not value:
+            return
+        if isinstance(value, MyWrapper):
+            return str(value)
+        return value
+
+
 class MyAutoField(models.BigAutoField):
     def from_db_value(self, value, expression, connection):
         if value is None:
diff --git a/tests/custom_pk/models.py b/tests/custom_pk/models.py
index 000ea7a29d..4ea5190d5f 100644
--- a/tests/custom_pk/models.py
+++ b/tests/custom_pk/models.py
@@ -7,7 +7,7 @@ this behavior by explicitly adding ``primary_key=True`` to a field.
 
 from django.db import models
 
-from .fields import MyAutoField, MyWrapperField
+from .fields import MyAutoField, MyWrapperField, MyWrapperFieldWithoutGetDbPrepValue
 
 
 class Employee(models.Model):
@@ -40,3 +40,9 @@ class Foo(models.Model):
 
 class CustomAutoFieldModel(models.Model):
     id = MyAutoField(primary_key=True)
+
+
+class CompositePKWithoutGetDbPrepValue(models.Model):
+    pk = models.CompositePrimaryKey(""id"", ""tenant"")
+    id = MyWrapperFieldWithoutGetDbPrepValue()
+    tenant = models.SmallIntegerField()
diff --git a/tests/custom_pk/tests.py b/tests/custom_pk/tests.py
index 5f865b680a..fa371942e6 100644
--- a/tests/custom_pk/tests.py
+++ b/tests/custom_pk/tests.py
@@ -2,7 +2,7 @@ from django.db import IntegrityError, transaction
 from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
 
 from .fields import MyWrapper
-from .models import Bar, Business, CustomAutoFieldModel, Employee, Foo
+from .models import Bar, Business, CompositePKWithoutGetDbPrepValue, CustomAutoFieldModel, Employee, Foo
 
 
 class BasicCustomPKTests(TestCase):
@@ -164,6 +164,14 @@ class CustomPKTests(TestCase):
         Business.objects.create(name=""Bears"")
         Business.objects.create(pk=""Tears"")
 
+    def test_custom_composite_pk_save_force_update(self):
+        """"""A custom primary key field that implements get_db_prep_save()
+        rather than get_db_prep_value() still saves.""""""
+        obj = CompositePKWithoutGetDbPrepValue()
+        obj.id = MyWrapper(""1"")
+        obj.tenant = 1
+        obj.save()  # passes with force_insert=True
+
     def test_unicode_pk(self):
         # Primary key may be Unicode string.
         Business.objects.create(name=""jaźń"")
}}}
----
{{{
ERROR: test_custom_composite_pk_save_force_update (custom_pk.tests.CustomPKTests.test_custom_composite_pk_save_force_update)
A custom primary key field that implements get_db_prep_save()
----------------------------------------------------------------------
Traceback (most recent call last):
  File ""/Users/.../django/django/db/backends/utils.py"", line 105, in _execute
    return self.cursor.execute(sql, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ""/Users/.../django/django/db/backends/sqlite3/base.py"", line 360, in execute
    return super().execute(query, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
sqlite3.ProgrammingError: Error binding parameter 3: type 'MyWrapper' is not supported

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File ""/Users/.../django/tests/custom_pk/tests.py"", line 173, in test_custom_composite_pk_save_force_update
    bar.save()  # passes with force_insert=True
    ^^^^^^^^^^
...

  File ""/Users/.../django/django/db/backends/sqlite3/base.py"", line 360, in execute
    return super().execute(query, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
django.db.utils.ProgrammingError: Error binding parameter 3: type 'MyWrapper' is not supported

----------------------------------------------------------------------
Ran 16 tests in 0.011s

FAILED (errors=1, skipped=1)
}}}
"	Bug	closed	Database layer (models, ORM)	dev	Release blocker	invalid		Csirmaz Bendegúz	Accepted	0	0	0	0	0	0