[GREEN] Change approach to use annotate (distinct not supported by test database)

apps/exportables/views.py

 from django.http import JsonResponse
+from django.db.models import Max
 from rest_framework import status
 from rest_framework.views import APIView
 
@@ -31,14 +32,19 @@ class StatisticsView(APIView):
     def get(self, request, format=None):
 
         case_subjects = CaseSubject.objects.all()
-        investigation_case_subjects = InvestigationCase.objects \
-            .order_by("case_subject__id", "-created_at") \
-            .distinct("case_subject")
+        investigation_cases = (
+            InvestigationCase.objects
+                .values("case_subject")
+                .annotate(latest_investigation_case=Max("created_at"))
+                .order_by()
+                .values("case_subject__id", "is_positive")
+        )
 
         outcomes = {}
-        for investigation_case_subject in investigation_case_subjects:
-            related_case_subject_id = investigation_case_subject.case_subject.id
-            outcomes[related_case_subject_id] = investigation_case_subject.is_positive
+
+        for investigation_case in investigation_cases:
+            related_case_subject_id = investigation_case["case_subject__id"]
+            outcomes[related_case_subject_id] = investigation_case["is_positive"]
 
         age_groups, sex_groups, district_groups = generate_initial_groups()