how to use django rest filtering with mongoengine for ListField

Question

I am trying to filter queryset for Many To Many Relationship. But its not working. I am trying to get objects of TestTag document with keyword name.

models :

class TestKeyword(Document):
    name = StringField(required=True)


class TestTag(Document):
    tag = StringField(max_length=100, null=True)  
    keywords = ListField(ReferenceField(TestKeyword), null=True)

Filter:

import django_mongoengine_filter as filters
from app.models import TestTag

class TestTagFilter(filters.FilterSet):
    class Meta:
        model = TestTag
        fields = ['tag', 'keywords__name']

class TestTag(ModelViewSet):
    queryset = TestTag.objects.all()
    serializer_class = TestTagSerializer
    # override filter_queryset function
    def filter_queryset(self, queryset):
        filter = TestTagFilter(self.request.query_params, queryset=queryset)
        return filter.qs

Answer 1

A general solution that worked for me is described bellow:

First make sure you have the following packages in your environment:

Django
djangorestframework
django-rest-framework-mongoengine
mongoengine
django-filter
# patched version of django-mongoengine-filter to support Django 4.0
# https://github.com/oussjarrousse/django-mongoengine-filter 
# Pull request https://github.com/barseghyanartur/django-mongoengine-filter/pull/16 or download the original if you are using Django 3.x
django-mongoengine-filter

The idea in this answer is to add filtering support to django-rest-framework-mongoengine using django-mongoengine-filter that is an replacement or an extension to django-filter and should work the same way as django-filter.

First let's edit the project/settings.py file. Find the INSTALLED_APPS variable and make sure the following "Django apps" are added:

# in settings.py:

INSTALLED_APPS = [
    # ...,
    "rest_framework",
    "rest_framework_mongoengine",
    "django_filters",
    # ...,
]

the app django_filters is required to add classes related to filtering infrastructure, and other things including html templates for DRF.

Then in the variable REST_FRAMEWORK we need to edit the values associated with the key: DEFAULT_FILTER_BACKENDS

# in settings.py:

REST_FRAMEWORK = {
    # ...
    "DEFAULT_FILTER_BACKENDS": [
        "filters.DjangoMongoEngineFilterBackend",
        # ...
    ],
    # ...
}

DjangoMongoEngineFilterBackend is a custom built filter backend that we need to add to the folder (depending on how you structure your project) in the file filters

# in filters.py:
from django_filters.rest_framework.backends import DjangoFilterBackend

class DjangoMongoEngineFilterBackend(DjangoFilterBackend):
    # filterset_base = django_mongoengine_filter.FilterSet
    """
    Patching the DjangoFilterBackend to allow for MongoEngine support
    """

    def get_filterset_class(self, view, queryset=None):
        """
        Return the `FilterSet` class used to filter the queryset.
        """
        filterset_class = getattr(view, "filterset_class", None)
        filterset_fields = getattr(view, "filterset_fields", None)

        if filterset_class:
            filterset_model = filterset_class._meta.model

            # FilterSets do not need to specify a Meta class
            if filterset_model and queryset is not None:
                element = queryset.first()
                if element:
                    queryset_model = element.__class__
                    assert issubclass(
                        queryset_model, filterset_model
                    ), "FilterSet model %s does not match queryset model %s" % (
                        filterset_model,
                        str(queryset_model),
                    )

            return filterset_class

        if filterset_fields and queryset is not None:
            MetaBase = getattr(self.filterset_base, "Meta", object)

            element = queryset.first()
            if element:
                queryset_model = element.__class__
                class AutoFilterSet(self.filterset_base):
                    class Meta(MetaBase):
                        model = queryset_model
                        fields = filterset_fields

            return AutoFilterSet

        return None

This custom filter backend will not raise the exceptions that the original django-filter filter backend would raise. The django-filter DjangoFilterBackend access the key model in QuerySet as in queryset.model, however that key does not exist in MongoEngine.

Maybe making it available in MongoEngine should be considered: https://github.com/MongoEngine/mongoengine/issues/2707 https://github.com/umutbozkurt/django-rest-framework-mongoengine/issues/294

Now we can add a custom filter to the ViewSet:

# in views.py
from rest_framework_mongoengine.viewsets import ModelViewSet

class MyModelViewSet(ModelViewSet):
    serializer_class = MyModelSerializer
    filter_fields = ["a_string_field", "a_boolean_field"]
    filterset_class = MyModelFilter

    def get_queryset(self):
        queryset = MyModel.objects.all()
        return queryset

Finally let's get back to filters.py and add the MyModelFilter

# in filters.py
from django_mongoengine_filter import FilterSet, StringField, BooleanField
class MyModelFilter(FilterSet):
    """
    MyModelFilter is a FilterSet that is designed to work with the django-filter.
    However the original django-mongoengine-filter is outdated and is causing some troubles
    with Django>=4.0.
    """

    class Meta:
        model = MyModel
        fields = [
            "a_string_field",
            "a_boolean_field",
        ]

    a_string_field = StringFilter()
    a_boolean_field = BooleanFilter()

That should do the trick.