Adding FlexFieldsFilterBackend immediately breaks the app

[striveforbest]

I am utilizing drf-flex-fields heavily and everything works fine until the moment i add:

REST_FRAMEWORK = {
    ...
    'DEFAULT_FILTER_BACKENDS': (
        'rest_flex_fields.filter_backends.FlexFieldsFilterBackend',
    ),
    ...
}

It crashes the app with the following traceback:

INFO 2020-03-03 21:38:35,864 autoreload 90410 4456287680 Watching for file changes with StatReloader
Performing system checks...

Exception in thread django-main-thread:
Traceback (most recent call last):
  File "/usr/local/Cellar/python@3.8/3.8.1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/threading.py", line 932, in _bootstrap_inner
    self.run()
  File "/usr/local/Cellar/python@3.8/3.8.1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/utils/autoreload.py", line 53, in wrapper
    fn(*args, **kwargs)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/core/management/commands/runserver.py", line 117, in inner_run
    self.check(display_num_errors=True)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/core/management/base.py", line 392, in check
    all_issues = self._run_checks(
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/core/management/base.py", line 382, in _run_checks
    return checks.run_checks(**kwargs)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/core/checks/registry.py", line 72, in run_checks
    new_errors = check(app_configs=app_configs)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/core/checks/urls.py", line 13, in check_url_config
    return check_resolver(resolver)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/core/checks/urls.py", line 23, in check_resolver
    return check_method()
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/urls/resolvers.py", line 407, in check
    for pattern in self.url_patterns:
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/utils/functional.py", line 48, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/urls/resolvers.py", line 588, in url_patterns
    patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/utils/functional.py", line 48, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/urls/resolvers.py", line 581, in urlconf_module
    return import_module(self.urlconf_name)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/Users/alex.zagoro/projects/gagosian/noya/gagosian/urls.py", line 6, in <module>
    path('api/v1/', include('gagosian.api.v1.urls', namespace='api'))
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/urls/conf.py", line 34, in include
    urlconf_module = import_module(urlconf_module)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/Users/alex.zagoro/projects/gagosian/noya/gagosian/api/v1/urls.py", line 4, in <module>
    from .contacts import urls as contacts_urls
  File "/Users/alex.zagoro/projects/gagosian/noya/gagosian/api/v1/contacts/urls.py", line 3, in <module>
    from .viewsets import (
  File "/Users/alex.zagoro/projects/gagosian/noya/gagosian/api/v1/contacts/viewsets.py", line 1, in <module>
    from rest_framework.viewsets import ModelViewSet
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/rest_framework/viewsets.py", line 27, in <module>
    from rest_framework import generics, mixins, views
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/rest_framework/generics.py", line 24, in <module>
    class GenericAPIView(views.APIView):
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/rest_framework/generics.py", line 43, in GenericAPIView
    filter_backends = api_settings.DEFAULT_FILTER_BACKENDS
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/rest_framework/settings.py", line 220, in __getattr__
    val = perform_import(val, attr)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/rest_framework/settings.py", line 168, in perform_import
    return [import_from_string(item, setting_name) for item in val]
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/rest_framework/settings.py", line 168, in <listcomp>
    return [import_from_string(item, setting_name) for item in val]
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/rest_framework/settings.py", line 177, in import_from_string
    return import_string(val)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/django/utils/module_loading.py", line 17, in import_string
    module = import_module(module_path)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/rest_flex_fields/__init__.py", line 3, in <module>
    from .views import FlexFieldsModelViewSet
  File "/Users/alex.zagoro/venv/noya-TS4FQGPC/lib/python3.8/site-packages/rest_flex_fields/views.py", line 21, in <module>
    class FlexFieldsModelViewSet(FlexFieldsMixin, viewsets.ModelViewSet):
AttributeError: partially initialized module 'rest_framework.viewsets' has no attribute 'ModelViewSet' (most likely due to a circular import)

Adding the backend directly to the view works fine.

[bharling]

Yeah seeing this too, but I get:

ImportError: Could not import 'rest_flex_fields.filter_backends.FlexFieldsFilterBackend' for API setting 'DEFAULT_FILTER_BACKENDS'. ImportError: cannot import name 'GenericViewSet'.

looks like a circular import also

[wolph]

As a workaround (I also posted this at #59 which appears to be a duplicate) you can set the filter backend at the view:

import rest_flex_fields.filter_backends as flex_filters
from rest_framework import viewsets


class ModelViewSet(viewsets.ModelViewSet):
    filter_backends = viewsets.ModelViewSet.filter_backends + [
        flex_filters.FlexFieldsFilterBackend,
    ]

[pablolmedorado]

Starting from @wolph idea, I have created a mixin to replace the "FlexFieldsMixin" that includes the FilterBackend and also respects the user's settings.

I'll leave the code here just in case there was anyone interested.

from rest_flex_fields.filter_backends import FlexFieldsFilterBackend
from rest_flex_fields.views import FlexFieldsMixin
from rest_framework.settings import api_settings


class FlexFieldsOptimizedMixin(FlexFieldsMixin):
    filter_backends = [FlexFieldsFilterBackend] + api_settings.DEFAULT_FILTER_BACKENDS

from rest_framework import viewsets
from .mixins import FlexFieldsOptimizedMixin


class ModelViewSet(FlexFieldsOptimizedMixin, viewsets.ModelViewSet):
    ...