Filtering django REST results by SerializerMethodField - django

Question

How would I filter results based on a computed field from a Serializer? I tried treating it like any other field, but django doesn't like it.
Serializer
class ImageSerializer(serializers.ModelSerializer):
is_annotated = serializers.SerializerMethodField('has_annotation')
class Meta:
model = Image
fields = '__all__'
#staticmethod
def has_annotation(image):
return image.annotation_set.count() > 0
View
class ImageViewSet(viewsets.ModelViewSet):
serializer_class = ImageSerializer
lookup_field = 'id'
permission_classes = [
Or(IsAdmin),
IsAuthenticated
]
def get_queryset(self):
queryset = Image.objects
is_annotated_filter = self.request.query_params.get('is_annotated', None)
if is_annotated_filter is not None:
queryset = queryset.filter.annotate(
cnt=Count('annotation')).filter(cnt__gte=1)
queryset_order = get_queryset_order(self.request)
return queryset.order_by(queryset_order).all()
http://www.django-rest-framework.org/api-guide/filtering/#filtering-against-query-parameters
http://www.django-rest-framework.org/api-guide/fields/#serializermethodfield

Answer 1

I think you misunderstand: 1) serializer MethodField which major purpose is READ-ONLY and the serializer is not about to be used in filtering queryset.
I will filter it more like these:
from django.db.models import Count
queryset.filter.annotate(
cnt=Count('annotation_set')
).filter(cnt__gte=1)
but... you can go even better:
1) just annotate your queryset e.g in your ViewSet
from django.db.models import Count
queryset = Image.objects.annotate(
cnt_annotations=Count('annotation_set')
)
2) then in serializer do something like these:
#staticmethod
def has_annotation(image):
if hasattr(obj, 'has_annotation'):
return bool(obj.cnt_annotations)
else:
return None