#86 Parcel-Geometry improvement

* improves the way parcel-geometry relations are stored on the DB
    * instead of a numerical sequence we switched to UUID, so no sequence will run out at anytime (new model: ParcelIntersection)
    * instead of dropping all M2M relations between parcel and geometry on each calculation, we keep the ones that still exist, drop the ones that do not exist and add new ones (if new ones exist)

konova/models/geometry.py

@@ -99,7 +99,7 @@ class Geometry(BaseResource):
         Returns:
 
         """
-        from konova.models import Parcel, District
+        from konova.models import Parcel, District, ParcelIntersection
         parcel_fetcher = ParcelWFSFetcher(
             geometry_id=self.id,
         )
@@ -107,6 +107,7 @@ class Geometry(BaseResource):
         fetched_parcels = parcel_fetcher.get_features(
             typename
         )
+        _now = timezone.now()
         underlying_parcels = []
         for result in fetched_parcels:
             fetched_parcel = result[typename]
@@ -125,19 +126,35 @@ class Geometry(BaseResource):
                 krs=fetched_parcel["ave:kreis"],
             )[0]
             parcel_obj.district = district
-            parcel_obj.updated_on = timezone.now()
+            parcel_obj.updated_on = _now
             parcel_obj.save()
             underlying_parcels.append(parcel_obj)
 
+        # Update the linked parcels
         self.parcels.set(underlying_parcels)
 
+        # Set the calculated_on intermediate field, so this related data will be found on lookups
+        intersections_without_ts = self.parcelintersection_set.filter(
+            parcel__in=self.parcels.all(),
+            calculated_on__isnull=True,
+        )
+        for entry in intersections_without_ts:
+            entry.calculated_on = _now
+        ParcelIntersection.objects.bulk_update(
+            intersections_without_ts,
+            ["calculated_on"]
+        )
+
     def get_underlying_parcels(self):
         """ Getter for related parcels and their districts
 
         Returns:
             parcels (QuerySet): The related parcels as queryset
         """
-        parcels = self.parcels.all().prefetch_related(
+
+        parcels = self.parcels.filter(
+            parcelintersection__calculated_on__isnull=False,
+        ).prefetch_related(
             "district"
         ).order_by(
             "gmrkng",