Database Features not set for Postgres for management commands

[valentijnscholten]

Hi,

Yes, I have added django-jsonfield-backport to INSTALLED_APPS.

Might be that the cause of the below is somewhere else / somewhere upstream in django, but I am seeing the following.
I have an application that works find with a json field on a model. I can delete that model just fine from the UI that have build, running with runserver or uwsgi etc. It also runs fine if I try to delete model instances from a management command.

When I run the same application with postgres and runserver or uwsgi, it also works fine. I can delete model instances.
But when I run a management command to delete model instances, I get an exception:

  File "./manage.py", line 11, in <module>
    execute_from_command_line(sys.argv)
  File "/home/valentijn/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/home/valentijn/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/valentijn/venv/lib/python3.8/site-packages/django/core/management/base.py", line 323, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/valentijn/venv/lib/python3.8/site-packages/django/core/management/base.py", line 364, in execute
    output = self.handle(*args, **options)
  File "/home/valentijn/dd/dojo/management/commands/test_valentijn5.py", line 10, in handle
    engagement.delete()
  File "/home/valentijn/dd/dojo/models.py", line 1042, in delete
    super().delete(*args, **kwargs)
  File "/home/valentijn/venv/lib/python3.8/site-packages/django/db/models/base.py", line 921, in delete
    collector.collect([self], keep_parents=keep_parents)
  File "/home/valentijn/venv/lib/python3.8/site-packages/django/db/models/deletion.py", line 224, in collect
    field.remote_field.on_delete(self, field, sub_objs, self.using)
  File "/home/valentijn/venv/lib/python3.8/site-packages/django/db/models/deletion.py", line 15, in CASCADE
    collector.collect(sub_objs, source=field.remote_field.model,
  File "/home/valentijn/venv/lib/python3.8/site-packages/django/db/models/deletion.py", line 223, in collect
    elif sub_objs:
  File "/home/valentijn/venv/lib/python3.8/site-packages/django/db/models/query.py", line 278, in __bool__
    self._fetch_all()
  File "/home/valentijn/venv/lib/python3.8/site-packages/django/db/models/query.py", line 1242, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/home/valentijn/venv/lib/python3.8/site-packages/django/db/models/query.py", line 72, in __iter__
    for row in compiler.results_iter(results):
  File "/home/valentijn/venv/lib/python3.8/site-packages/django/db/models/sql/compiler.py", line 1086, in apply_converters
    value = converter(value, expression, connection)
  File "/home/valentijn/venv/lib/python3.8/site-packages/django_jsonfield_backport/models.py", line 111, in from_db_value
    if connection.features.has_native_json_field and self.decoder is None:
AttributeError: 'DatabaseFeatures' object has no attribute 'has_native_json_field'

When debugging, it looks like the extend_features is not called in this scenario. Looking at the code of this backport it connects to the connection_created signal. For some reason this doesn't seem to be called when running as a management command against a postgres database. For MySQL it runs fine. As said, might be something upstream, but at least I can share my workaround here. I added the below to my management command to make it work:

from django_jsonfield_backport.features import extend_features
from django.db import connection

class Command(BaseCommand):

    def handle(self, *args, **options):
        extend_features(connection)
     
        .... do_stuff .... 

[laymonage]

Hi @valentijnscholten, thanks for the report! I'm afraid that's not something I can handle from my side. Maybe try creating a ticket to Django? The workaround seems logical, though. Thanks for sharing!

[josterpi]

I am using MySQL and have run into this as well. In addition to management commands, ./manage.py shell also has this problem.

I dug into it and found that the connection_created signal is sent before extend_features gets registered in those cases, so it never ends up running.

I've created a pull request that calls extend_features(connection) if connection.is_usable() or registers the signal handler otherwise.

[laymonage]

Fixed in v1.0.5, please verify. Thanks!

[valentijnscholten]

For the record, it's fixed in #39