TypeError, Direct assignment to the reverse side of a related set is prohibited. Use +.set() instead

[Mike Schmidt]

I created a new mezzanine project with the following software versions:

    Linux 4.18.17-041817-generic (Ubuntu 18.04 with newer kernel)

    MariaDB 10.1.34 (running fine after explicitly creating the database with utf8 character encoding)

    Python 3.6.6

    Django 2.0.9 (from pypi)

    Mezzanine 4.3.1 (from github, along with filebrowser-safe)

The only modifications I've made are changing the timezone in settings.py and changing the database dictionary in local_settings.py.

Everything seemed to work. 'python manage.py createdb' was successful, then 'python manage.py runserver 127.0.0.1:8000' was also successful.

I logged into the admin interface, navigated to add a new richtext page, filled in the title and body, and clicked save. A debug-friendly error page was then returned, and the following text was in the console.

    [12/Nov/2018 14:28:08] "POST /admin/admin_keywords_submit/ HTTP/1.1" 200 1

    Internal Server Error: /admin/pages/richtextpage/add/

    Traceback (most recent call last):

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/core/handlers/exception.py", line 35, in inner

        response = get_response(request)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/core/handlers/base.py", line 128, in _get_response

        response = self.process_exception_by_middleware(e, request)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/core/handlers/base.py", line 126, in _get_response

        response = wrapped_callback(request, *callback_args, **callback_kwargs)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/contrib/admin/options.py", line 575, in wrapper

        return self.admin_site.admin_view(view)(*args, **kwargs)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/utils/decorators.py", line 142, in _wrapped_view

        response = view_func(request, *args, **kwargs)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/views/decorators/cache.py", line 44, in _wrapped_view_func

        response = view_func(request, *args, **kwargs)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/contrib/admin/sites.py", line 223, in inner

        return view(request, *args, **kwargs)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/mezzanine/pages/admin.py", line 66, in add_view

        return super(PageAdmin, self).add_view(request, **kwargs)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/contrib/admin/options.py", line 1554, in add_view

        return self.changeform_view(request, None, form_url, extra_context)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/utils/decorators.py", line 62, in _wrapper

        return bound_func(*args, **kwargs)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/utils/decorators.py", line 142, in _wrapped_view

        response = view_func(request, *args, **kwargs)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/utils/decorators.py", line 58, in bound_func

        return func.__get__(self, type(self))(*args2, **kwargs2)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/contrib/admin/options.py", line 1451, in changeform_view

        return self._changeform_view(request, object_id, form_url, extra_context)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/contrib/admin/options.py", line 1492, in _changeform_view

        self.save_related(request, form, formsets, not add)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/contrib/admin/options.py", line 1049, in save_related

        form.save_m2m()

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/forms/models.py", line 439, in _save_m2m

        f.save_form_data(self.instance, cleaned_data[f.name])

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/mezzanine/generic/fields.py", line 206, in save_form_data

        super(KeywordsField, self).save_form_data(instance, data)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/db/models/fields/__init__.py", line 838, in save_form_data

        setattr(instance, self.name, data)

      File "/home/mike/projects/plab/venv/lib/python3.6/site-packages/django/db/models/fields/related_descriptors.py", line 509, in __set__

        % self._get_set_deprecation_msg_params(),

    TypeError: Direct assignment to the reverse side of a related set is prohibited. Use +.set() instead.

    [12/Nov/2018 14:28:08] "POST /admin/pages/richtextpage/add/ HTTP/1.1" 500 146350

Attempting to debug this in pycharm, I found that the item being saved was a 'keyword' of ''. I had not entered any keywords in the form, so it appears the empty string is saved, anyway. I've read through both django and mezzanine code in the callstack, and it seems like this is related to django's database layer and ManyToMany relationships. But it seems the ultimate saving of the empty keyword is a Field.setattr() call, which is neither an = or a .set(). And I don't know what the '+' character is from.

Can anyone help me to understand what needs to be fixed, here? Since I haven't actually written any code, I'm tempted to think either mezzanine or django needs a pull request. Or perhaps there's a compatibility issue between versions of django and mezzanine? I haven't seen anything obvious in git diffs. But until I understand better, I wouldn't know how to go about fixing it. I have found a few slightly similar things in this group and on stackexchange, but nothing that matches. The same behavior occurs if I run the server and try to add a blog post rather than a richtextpage. And I get the same behavior on a different server running debian 8 rather than ubuntu 18.

Thanks for any advice!

[Stephen McDonald]

There's an open issue for this: https://github.com/stephenmcd/mezzanine/issues/1899

I believe the GitHub version of Mezzanine had the maximum supported Django version changed to 2.0 prematurely, although there's been a ton of compatibility work done and the issue you're looking at is the last known problem with Django 2.0. Once that's resolved we can do a new official release supporting 2.0.

I don't have any insight to the cause of the issue nor any immediate availability to look at it myself.

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Stephen McDonald
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[Mike Schmidt]

Thank you for your reply, and I'm sorry I didn't see the open issue before posting.

I can't promise a resolution, but I'll continue looking and testing.

For further information, this same issue also occurs with postgresql, as I'd expect if it's occurring where I think it is. And I'm able to save site settings to the database, just not new blogs or richtextpages.

Cheers,

    Mike