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 ​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 ​logic to add `pk` to the list of fields, so I wonder if it's not being prepared the same way in SQLUpdateCompiler?

tests/custom_pk/fields.py

@@ -59 +59 @@
         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:

tests/custom_pk/models.py

@@ -7 +7 @@
 
 from django.db import models
 
-from .fields import MyAutoField, MyWrapperField
+from .fields import MyAutoField, MyWrapperField, MyWrapperFieldWithoutGetDbPrepValue
 
 
 class Employee(models.Model):
@@ -40 +40 @@
 
 class CustomAutoFieldModel(models.Model):
     id = MyAutoField(primary_key=True)
+
+
+class CompositePKWithoutGetDbPrepValue(models.Model):
+    pk = models.CompositePrimaryKey("id", "tenant")
+    id = MyWrapperFieldWithoutGetDbPrepValue()
+    tenant = models.SmallIntegerField()

tests/custom_pk/tests.py

@@ -2 +2 @@
 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 +164 @@
         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)