Migration test failure on Oracle

[Shai Berger]

On CI, reporoduces also on my laptop:

======================================================================
ERROR [0.000s]: test_run_python_related_assignment (migrations.test_operations.OperationTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ".../tests/migrations/test_operations.py", line 1714, in test_run_python_related_assignment
    create_old_man.database_forwards("test_books", editor, project_state, new_state)
  File ".../django/db/migrations/operations/special.py", line 179, in database_forwards
    self.code(from_state.apps, schema_editor)
  File ".../tests/migrations/test_operations.py", line 1671, in inner_method
    author = Author.objects.create(name="Hemingway")
  File ".../django/db/models/manager.py", line 127, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File ".../django/db/models/query.py", line 401, in create
    obj.save(force_insert=True, using=self.db)
  File ".../django/db/models/base.py", line 676, in save
    force_update=force_update, update_fields=update_fields)
  File ".../django/db/models/base.py", line 704, in save_base
    updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
  File ".../django/db/models/base.py", line 788, in _save_table
    result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
  File ".../django/db/models/base.py", line 827, in _do_insert
    using=using, raw=raw)
  File ".../django/db/models/manager.py", line 127, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File ".../django/db/models/query.py", line 1026, in _insert
    return query.get_compiler(using=using).execute_sql(return_id)
  File ".../django/db/models/sql/compiler.py", line 963, in execute_sql
    cursor.execute(sql, params)
  File ".../django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File ".../django/db/utils.py", line 92, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File ".../django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File ".../django/db/backends/oracle/base.py", line 478, in execute
    return self.cursor.execute(query, self._param_generator(params))
IntegrityError: ORA-01400: cannot insert NULL into ("D_DJANGOORACLE_27"."TEST_AUTHORS_AUTHOR"."ID")

Bisecting leads to the commit that added the test; it was added (failing) for #24282, and the failure was later fixed.
The failure on Oracle is unrelated to the problem reported on the ticket, but since the solution (b29f3b51204d53c1c8745966476543d068c173a2) involved a rewrite of model reloading, it may be related.

[Tim Graham]

​PR -- Hopefully you can explain it.

[Markus Holtermann]

The only thing how I'd be to explain this problem is by some statement cache on oracle, but not sure if this applies here or anymore in general. It's definitely unrelated to the state rewrite.

[Shai Berger <shai@…>]

In f7d34fe741ea64f3ec89f1d742d47182065e2c2a:

Fixed #24362: Made test run each migration operation in its own schema editor

Thanks Tim Graham for pointing out the fix.

[Shai Berger <shai@…>]

In e2a3be1e4d9a85aa30f1b7d9064b5abb1569eea6:

[1.8.x] Fixed #24362: Made test run each migration operation in its own schema editor

Thanks Tim Graham for pointing out the fix.

Backport of f7d34fe from master ticket_24362

[Shai Berger]

The explanation is that on Oracle, the auto-field is implemented using a trigger; the code that creates the trigger is added in the CreateModel migration operation as deferred sql. Deferred sql is only executed in the schema editor's __exit__. In most tests around this one in the file you can see migration operations performed each in its own schema editor. I did the same here.