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
36086	Creating an instance of a model with a non-auto primary key and a generated field crashes on backends not supporting returning columns from inserts	Simon Charette	Simon Charette	"Discovered while working on #36075.

---

The `SQLInsertCompiler.execute_sql` changes (7254f1138d9c51fa558229c39c9559b369c4278a) made to accommodate `returning_fields` support (#29444) failed to account for the possibility that backends not supporting returning columns from inserts (MySQL, SQLite < 3.35, and Oracle when the `use_returning_into` option is disabled) could still request `returning_fields` and not have an auto-primary key when `GeneratedField` was introduced.

Given the following model

{{{#!python
class TestModel(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4)
    a = models.IntegerField()
    b = models.GeneratedField(
        expression=F(""a""),
        output_field=models.IntegerField(),
        db_persist=True,
    )
}}}

Attempting to perform `TestModel.objects.create(a=1)` crashes because even if there is no usage of `AutoField` [https://github.com/django/django/blob/8bee7fa45cd7bfe70b68784314e994e2d193fd70/django/db/models/sql/compiler.py#L1926-L1937 we still attempt to] call `last_insert_id` and for the `UUIDField` primary key (which will either return `0` or crash on Oracle since it attempts to retrieve a non-existing sequence) and then try to convert `0` to a `uuid.UUID` which crashes with


{{{
Traceback (most recent call last):
  File ""/django/source/tests/model_fields/test_generatedfield.py"", line 361, in test_create_with_non_auto_pk
    obj = GeneratedModelNonAutoPk.objects.create(a=1, b=2)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ""/django/source/django/db/models/manager.py"", line 87, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ""/django/source/django/db/models/query.py"", line 663, in create
    obj.save(force_insert=True, using=self.db)
  File ""/django/source/django/db/models/base.py"", line 901, in save
    self.save_base(
  File ""/django/source/django/db/models/base.py"", line 1007, in save_base
    updated = self._save_table(
              ^^^^^^^^^^^^^^^^^
  File ""/django/source/django/db/models/base.py"", line 1168, in _save_table
    results = self._do_insert(
              ^^^^^^^^^^^^^^^^
  File ""/django/source/django/db/models/base.py"", line 1209, in _do_insert
    return manager._insert(
           ^^^^^^^^^^^^^^^^
  File ""/django/source/django/db/models/manager.py"", line 87, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ""/django/source/django/db/models/query.py"", line 1845, in _insert
    return query.get_compiler(using=using).execute_sql(returning_fields)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ""/django/source/django/db/models/sql/compiler.py"", line 1927, in execute_sql
    return list(rows)
           ^^^^^^^^^^
  File ""/django/source/django/db/models/sql/compiler.py"", line 1541, in apply_converters
    value = converter(value, expression, connection)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ""/django/source/django/db/backends/mysql/operations.py"", line 327, in convert_uuidfield_value
    value = uuid.UUID(value)
            ^^^^^^^^^^^^^^^^
  File ""/usr/local/lib/python3.11/uuid.py"", line 175, in __init__
    hex = hex.replace('urn:', '').replace('uuid:', '')
          ^^^^^^^^^^^
AttributeError: 'int' object has no attribute 'replace'
}}}

Note that in a case of a non-integer primary, like described above we get a crash, but in a case of an integer one the issue is even more insidious as the bogus value (which will be either be `0` or whatever value the last inserted table with an auto primary key was) will happily pass the conversion layer and then be wrongly assigned to `GeneratedField` value. In other words, if the model is defined like the following instead

{{{#!python
class TestModel(models.Model):
    id = models.IntegerField(primary_key=True)
    a = models.IntegerField()
    b = models.GeneratedField(
        expression=F(""a""),
        output_field=models.IntegerField(),
        db_persist=True,
    )
}}}

Then we'll encounter the following problem

{{{#!python
>>> obj = TestModel.object.create(id=1, a=42)
>>> obj.b
1  # This should be 42!!
}}} "	Bug	closed	Database layer (models, ORM)	5.0	Normal	fixed			Ready for checkin	1	0	0	0	0	0