Django Rest Framework filtering calculated model property

Question

Sorry for a newbie question. I have a following model:

class WeightSlip(models.Model):

    grossdate = models.DateTimeField(auto_now=False, auto_now_add=False)
    grossweight = models.DecimalField(max_digits=6, decimal_places=2, default=0)
    taredate = models.DateTimeField(auto_now=False, auto_now_add=False)
    tareweight = models.DecimalField(max_digits=6, decimal_places=2, default=0)
    vehicle = models.CharField(max_length=12)

    @property
    def netweight(self):
        return self.grossweight - self.tareweight

    @property
    def slipdate(self):
        if self.grossdate > self.taredate:
           return grossdate.date()
        else:
           return taredate.date()

Serializer:

class WeightSlipSerializer(serializers.ModelSerializer):

   class Meta:
      model = models.WeightSlip
      fields = ('grossdate', 'grossweight', 'taredate', 'tareweight', 'slipdate', 'netweight', 'vehicle')
      read_only_fields = ('slipdate', 'netweight')

I am trying to use the django-rest-framework-filters to filter on the calculated 'netweight' and 'slipdate' properties:

class WeightSlipFilter(FilterSet):

   class Meta:
       model = WeightSlip
       fields = ('slipdate', 'netweight', 'vehicle')

This gives me an error:

TypeError: 'Meta.fields' contains fields that are not defined on this FilterSet: slipdate, netweight

Is there a workaround to this problem other than adding the calculated fields to the database ?

Thanks in advance.

Answer 1

You can create custom filter for slipdate, netweight that will evaluate and filter this fields in db. For this you can use conditional expressions and F expression

from django.db.models import F, Case, When

class WeightSlipFilter(FilterSet):
    slipdate = DateTimeFilter(method='filter_slipdate')
    netweight = NumberFilter(method='filter_netweight')

    class Meta:
        model = WeightSlip
        fields = ('slipdate', 'netweight', 'vehicle')

    def filter_netweight(self, queryset, value):
        if value:
            queryset = queryset.annotate(netweight=F('grossweight') - F('tareweight')).filter(netweight=value)
        return queryset

    def filter_slipdate(self, queryset, value):
        if value:
            queryset = queryset.annotate(slipdate=Case(When(grossdate__gt=F('taredate'), then=F('grossdate')), default=F('taredate')).filter(slipdate=value)
        return queryset

Answer 2

Note that if you're using the latest version of django-filter, Filter.method takes 4 arguments, like so:

def filter_slipdate(self, queryset, name, value):
    if value:
        queryset = queryset.annotate(slipdate=Case(When(grossdate__gt=F('taredate'), then=F('grossdate')), default=F('taredate')).filter(slipdate=value)
    return queryset

```