cannot convert dictionary update sequence element #0 to a sequence

[shimulch]

One line description of the issue

ArrayReferenceManager.add() method is throwing following error if called right after ArrayReferenceManager.clear().

Python script (Example)

class Example(models.Model):
    submodels = models.ArrayReferenceField(SubModel, blank=True)

# Somewhere in code
example.submodels.clear()
example.submodels.add(subModelInstance)

Traceback

Environment:

Request Method: POST
Request URL: http://127.0.0.1:8000/dashboard/data/edit/?id=5bb9a8a4d3e950069a550c83&wizard=files

Django Version: 2.1.2
Python Version: 3.6.6
Installed Applications:
['johukum',
 'widget_tweaks',
 'treebeard',
 'formtools',
 'django_countries',
 'django_extensions',
 'django_select2',
 'django_tables2',
 'django.contrib.admin',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'django.contrib.humanize',
 'rest_framework',
 'rest_framework.authtoken',
 'django_filters']
Installed Middleware:
['django.middleware.security.SecurityMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.common.CommonMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'django.middleware.clickjacking.XFrameOptionsMiddleware']

Traceback:

File "/Users/shimul/Projects/johukum/.venv/lib/python3.6/site-packages/django/core/handlers/exception.py" in inner
  34.             response = get_response(request)

File "/Users/shimul/Projects/johukum/.venv/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
  126.                 response = self.process_exception_by_middleware(e, request)

File "/Users/shimul/Projects/johukum/.venv/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
  124.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/Users/shimul/Projects/johukum/.venv/lib/python3.6/site-packages/django/views/generic/base.py" in view
  68.             return self.dispatch(request, *args, **kwargs)

File "/Users/shimul/Projects/johukum/.venv/lib/python3.6/site-packages/django/views/generic/base.py" in dispatch
  88.         return handler(request, *args, **kwargs)

File "/Users/shimul/Projects/johukum/.venv/lib/python3.6/site-packages/django/views/generic/edit.py" in post
  142.             return self.form_valid(form)

File "/Users/shimul/Projects/johukum/johukum/views.py" in form_valid
  236.             form.save(obj, update=True)

File "/Users/shimul/Projects/johukum/johukum/forms.py" in save
  353.             business_info.videos.add(uploaded_video)

File "/Users/shimul/Projects/johukum/.venv/lib/python3.6/site-packages/djongo/models/fields.py" in add
  858.             fks.update(new_fks)

Exception Type: TypeError at /dashboard/data/edit/
Exception Value: cannot convert dictionary update sequence element #0 to a sequence

[shimulch]

After some digging I think the issue lies in djongo/models/fields.py:908

# Default value is set to be a dict
setattr(self.instance, self.field.attname, {})

# It should be a Set
setattr(self.instance, self.field.attname, set())

If you think this is what it should be, I can do a PR request.

[shimulch]

192