How to use ModelMultipleChoiceFilter?

Question

I have been trying to get a ModelMultipleChoiceFilter to work for hours and have read both the DRF and Django Filters documentation.

I want to be able to filter a set of Websites based on the tags that have been assigned to them via a ManyToManyField. For example I want to be able to get a list of websites that have been tagged "Cooking" or "Beekeeping".

Here is the relevant snippet of my current models.py:

class SiteTag(models.Model):
    """Site Categories"""
    name = models.CharField(max_length=63)

    def __str__(self):
        return self.name

class Website(models.Model):
    """A website"""
    domain = models.CharField(max_length=255, unique=True)
    description = models.CharField(max_length=2047)
    rating = models.IntegerField(default=1, choices=RATING_CHOICES)
    tags = models.ManyToManyField(SiteTag)
    added = models.DateTimeField(default=timezone.now())
    updated = models.DateTimeField(default=timezone.now())

    def __str__(self):
        return self.domain

And my current views.py snippet:

class WebsiteFilter(filters.FilterSet):
    # With a simple CharFilter I can chain together a list of tags using &tag=foo&tag=bar - but only returns site for bar (sites for both foo and bar exist).
    tag = django_filters.CharFilter(name='tags__name')

    # THE PROBLEM:
    tags = django_filters.ModelMultipleChoiceFilter(name='name', queryset=SiteTag.objects.all(), lookup_type="eq")

    rating_min = django_filters.NumberFilter(name="rating", lookup_type="gte")
    rating_max = django_filters.NumberFilter(name="rating", lookup_type="lte")

    class Meta:
        model = Website
        fields = ('id', 'domain', 'rating', 'rating_min', 'rating_max', 'tag', 'tags')

class WebsiteViewSet(viewsets.ModelViewSet):
    """API endpoint for sites"""
    queryset = Website.objects.all()
    serializer_class = WebsiteSerializer
    filter_class = WebsiteFilter
    filter_backends = (filters.DjangoFilterBackend, filters.SearchFilter, filters.OrderingFilter,)
    search_fields = ('domain',)
ordering_fields = ('id', 'domain', 'rating',)

I have just been testing with the querystring [/path/to/sites]?tags=News and I am 100% sure that the appropriate records exist as they work (as described) with a ?tag (missing the s) query.

An example of the other things I have tried is something like:

tags = django_filters.ModelMultipleChoiceFilter(name='tags__name', queryset=Website.objects.all(), lookup_type="in")

How can I return any Website that has a SiteTag that satisfies name == A OR name == B OR name == C?

Answer 1

I stumbled across this question while trying to solve a nearly identical problem to yourself, and while I could have just written a custom filter, your question got me intrigued and I had to dig deeper!

It turns out that a ModelMultipleChoiceFilter only makes one change over a normal Filter, as seen in the django_filters source code below:

class ModelChoiceFilter(Filter):
    field_class = forms.ModelChoiceField

class ModelMultipleChoiceFilter(MultipleChoiceFilter):
    field_class = forms.ModelMultipleChoiceField

That is, it changes the field_class to a ModelMultipleChoiceField from Django's built in forms.

Taking a look at the source code for ModelMultipleChoiceField, one of the required arguments to __init__() is queryset, so you were on the right track there.

The other piece of the puzzle comes from the ModelMultipleChoiceField.clean() method, with a line: key = self.to_field_name or 'pk'. What this means is that by default it will take whatever value you pass to it (eg.,"cooking") and try to look up Tag.objects.filter(pk="cooking"), when obviously we want it to look at the name, and as we can see in that line, what field it compares to is controlled by self.to_field_name.

Luckily, django_filters's Filter.field() method includes the following when instantiating the actual field.

self._field = self.field_class(required=self.required,
    label=self.label, widget=self.widget, **self.extra)

Of particular note is the **self.extra, which comes from Filter.__init__(): self.extra = kwargs, so all we need to do is pass an extra to_field_name kwarg to the ModelMultipleChoiceFilter and it will be handed through to the underlying ModelMultipleChoiceField.

So (skip here for the actual solution!), the actual code you want is

tags = django_filters.ModelMultipleChoiceFilter(
    name='sitetags__name',
    to_field_name='name',
    lookup_type='in',
    queryset=SiteTag.objects.all()
)

So you were really close with the code you posted above! I don't know if this solution will be relevant to you anymore, but hopefully it might help someone else in the future!