UUIDField support: How does it work?

[nockty]

Subject: When using UUIDField: django.db.utils.ProgrammingError: type "uuid" does not exist

Problem

• Redshift doesn't support uuid data type but django-redshift-backend's documentation states that UUIDField is supported by using another type mapping
• It turns out I don't figure out how to make it work.

Procedure to reproduce the problem

• Create a model which uses django.db.models.UUIDField

class SomeModel:
    uuid = models.UUIDField(default=uuid.uuid4, editable=False)
    # some other fields

• Try to save an instance of the model.

>>> some_instance = SomeModel()
>>> some_instance.save(using='some_database')

• My database uses django-redshift-backend:

>>> django.db.connections['some_database']
<django_redshift_backend.base.DatabaseWrapper object at 0x7f17ee6f35c0>
>>> django.db.connections['some_database'].data_types['UUIDField']
'varchar(32)'

Error logs / results

• Traceback:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/local/lib/python3.6/site-packages/django/db/models/base.py", line 718, in save
    force_update=force_update, update_fields=update_fields)
  File "/usr/local/lib/python3.6/site-packages/django/db/models/base.py", line 748, in save_base
    updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
  File "/usr/local/lib/python3.6/site-packages/django/db/models/base.py", line 831, in _save_table
    result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
  File "/usr/local/lib/python3.6/site-packages/django/db/models/base.py", line 869, in _do_insert
    using=using, raw=raw)
  File "/usr/local/lib/python3.6/site-packages/django/db/models/manager.py", line 82, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/django/db/models/query.py", line 1136, in _insert
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/usr/local/lib/python3.6/site-packages/django/db/models/sql/compiler.py", line 1289, in execute_sql
    cursor.execute(sql, params)
  File "/usr/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 100, in execute
    return super().execute(sql, params)
  File "/usr/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 68, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/usr/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 77, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/usr/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
  File "/usr/local/lib/python3.6/site-packages/django/db/utils.py", line 89, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/usr/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: type "uuid" does not exist

• Raw SQL query:

(0.167) INSERT INTO "someapp_somemodel" ("field1", "field2", "field3", "field4", "field5", "field6", "field7", "field8", "field9", "uuid", "field11", "field12", "field13", "field14") VALUES ('', NULL, NULL, NULL, NULL, NULL, NULL, NULL, 'unk', '7a6b990a-c24b-4146-adb7-986b3fe7724d'::uuid, '', 'json', NULL, 2);

• Corresponding Python arguments:

args=['', None, None, None, None, None, None, None, 'unk', UUID('7a6b990a-c24b-4146-adb7-986b3fe7724d'), '', 'json', None, 2]

Expected results

I would expect that the uuid data type is not used when saving the instance in the database. Then again, maybe I'm doing things wrong and I completely miss something important when trying to use UUIDField. In this case, maybe the documentation should be updated to teach users how to use it?

Environment info

• OS: Docker container built by using the python:3.6 image
• Python version: 3.6.6
• Django version: 2.1.3 (I know django-redshift-backend is not supposed to support Django2.1 yet, but then why is such an issue happening anyway?)
• Django-Redshift-Backend version: 0.9.1