Add a field to DRF generic list view - django

Question

I need to create a DRF list view that shows each course along with a boolean field signifying whether the user requesting the view is subscribed to the course.
Course subscriptions are stored in the following model:
class Subscription(models.Model):
user = models.ForeignKey(
CustomUser, related_name='subscriptions', null=False,
on_delete=CASCADE)
course = models.ForeignKey(
Course, related_name='subscriptions', null=False,
on_delete=CASCADE)
class Meta:
ordering = ['course', 'user']
unique_together = [['user', 'course']]
This is the view I am writing:
class CourseListView(generics.ListAPIView):
permission_classes = [IsAuthenticated, ]
queryset = Course.objects.all()
serializer_class = CourseSerializer
def isSubscribed(self, request, course):
sub = Subscription.objects.filter(
user=request.user, course=course).first()
return True if sub else False
def list(self, request, format=None):
queryset = Course.objects.all()
serializer = CourseSerializer(queryset, many=True)
return Response(serializer.data)
I am looking for a way to modify the list method, so as to add to the response the information about whether request.user is subscribed to each of the courses.
The best solution I have for now is to construct serializer manually, which would look (at the level of pseudo-code) something like this:
serializer = []
for course in querySet:
course['sub'] = self.isSubscribed(request, course)
serializer.append(CourseSerializer(course))
I suspect there should be a better (standard, idiomatic, less convoluted) way for adding a custom field in a list view, but could not find it. In addition, I am wondering whether it is possible to avoid a database hit for every course.

Answer 1

You can do that easily with Exists:
just change your queryset in your view:
from django.db.models import Exists, OuterRef
class CourseListView(generics.ListAPIView):
permission_classes = [IsAuthenticated, ]
serializer_class = CourseSerializer
def get_queryset(self):
subquery = Subscription.objects.filter(user=request.user, course=OuterRef('id'))
return Course.objects.annotate(sub=Exists(subquery))
and add a field for it in your serializer:
class CourseSerializer(serializers.ModelSerializer):
sub = serializers.BooleanField(read_only=True)
class Meta:
model = Course
fields = '__all__'