DRF Vanilla view serializer

serializers.py

import json
class UserSerializer():
    model = User
    fields = ['first_name', 'last_name']
    def get_queryset(self):
        return self.model.objects.all()
    @property
    def data(self):
        users = self.get_queryset()
        for user in users:
            groups = list(user.groups.values('id', 'name'))
            user['groups'] = groups
            user['is_birthday_soon'] = user.is_birthday_in_this_month()
        return json.dumps(users)