reduce number of repeated queries

move queries that get favourites and auto converted to methods that
generate dicts hence one query for all rather than one query per data
set

hub/views/area.py

@@ -1,6 +1,6 @@
 from collections import defaultdict
 
-from django.db.models import Count, OuterRef, Subquery
+from django.db.models import Count
 from django.http import JsonResponse
 from django.shortcuts import get_object_or_404, redirect
 from django.views.generic import DetailView, TemplateView, View
@@ -47,23 +47,31 @@ def get_object(self):
     def get_page_title(self):
         return self.object.name
 
-    def process_dataset(self, data_set):
-        fav_sq = Subquery(
+    def get_user_favourite_datasets(self):
+        favs = (
             UserDataSets.objects.filter(
-                data_set_id=OuterRef("data_type__data_set__id"),
                 user=self.request.user,
             )
             .values("data_set_id")
             .annotate(is_favourite=Count("id"))
             .values("is_favourite")
         )
-        data_type = DataType.objects.filter(
-            data_set=data_set, area_type=self.object.area_type
-        ).first()
-        auto_converted = False
-        if data_type is not None:
-            auto_converted = data_type.auto_converted
 
+        fav_map = {}
+        for fav in favs:
+            fav_map["fav.data_set_id"] = fav["is_favourite"]
+
+        return fav_map
+
+    def get_auto_convered_datasets(self):
+        auto_converted = {}
+
+        for ds in DataType.objects.filter(area_type=self.object.area_type):
+            auto_converted[ds.data_set_id] = ds.auto_converted
+
+        return auto_converted
+
+    def process_dataset(self, data_set, favs, auto_converted):
         base_qs = AreaData.objects.filter(
             area=self.object,
             data_type__data_set=data_set,
@@ -81,33 +89,26 @@ def process_dataset(self, data_set):
             "data_type": data_set.data_type,
             "featured": data_set.featured,
             "excluded_countries": data_set.exclude_countries,
-            "auto_converted": auto_converted,
+            "auto_converted": auto_converted.get(data_set.id, False),
+            "is_favourite": favs.get(data_set.id, False),
         }
         if data_set.is_range:
             data["is_range"] = True
             data_range = base_qs.select_related("data_type").order_by("data_type__name")
 
-            data["is_favourite"] = UserDataSets.objects.filter(
-                data_set=data_set,
-                user=self.request.user,
-            ).exists()
             d = data_range.all()
             if len(d) == 0:
                 d = None
 
             data["data"] = d
         elif data_set.category == "opinion":
-            data_range = (
-                base_qs.annotate(is_favourite=fav_sq)
-                .select_related("data_type")
-                .order_by("data_type__order", "data_type__label")
+            data_range = base_qs.select_related("data_type").order_by(
+                "data_type__order", "data_type__label"
             )
 
             data["data"] = data_range.all()
         else:
-            area_data = base_qs.annotate(is_favourite=fav_sq).select_related(
-                "data_type"
-            )
+            area_data = base_qs.select_related("data_type")
             if area_data:
                 data["data"] = area_data[0]
 
@@ -190,10 +191,12 @@ def get_context_data(self, **kwargs):
 
         categories = defaultdict(list)
         indexed_categories = defaultdict(dict)
+        favs = self.get_user_favourite_datasets()
+        auto_converted = self.get_auto_convered_datasets()
         for data_set in DataSet.objects.order_by("order", "label").filter(
             areas_available=self.object.area_type
         ):
-            data = self.process_dataset(data_set)
+            data = self.process_dataset(data_set, favs, auto_converted)
 
             if data.get("data", None) is not None and data["data"]:
                 if data_set.category is not None: