Use already-transformed polygons wherever possible

Transforming full resolution polygons to linear coordinates is somewhat
expensive. We can make things run faster by:
- looking at `simple_polygons` which have fewer points and are therefore
  faster to transform.
- reusing polygons that have already been transformed where possible

app/broadcast_areas/models.py

@@ -61,9 +61,9 @@ class BaseBroadcastArea(ABC):
 
     @cached_property
     def phone_density(self):
-        if not self.polygons.estimated_area:
+        if not self.simple_polygons.estimated_area:
             return 0
-        return self.count_of_phones / square_metres_to_square_miles(self.polygons.estimated_area)
+        return self.count_of_phones / square_metres_to_square_miles(self.simple_polygons.estimated_area)
 
     @property
     def estimated_bleed_in_m(self):
@@ -160,9 +160,11 @@ class CustomBroadcastArea(BaseBroadcastArea):
 
     @classmethod
     def from_polygon_objects(cls, polygon_objects):
-        return cls(name=None, polygons=polygon_objects.as_coordinate_pairs_lat_long)
+        instance = cls(name=None)
+        instance.polygons = polygon_objects
+        return instance
 
-    @property
+    @cached_property
     def polygons(self):
         return Polygons(
             # Polygons in the DB are stored with the coordinate pair
@@ -189,7 +191,7 @@ class CustomBroadcastArea(BaseBroadcastArea):
         return broadcast_area_libraries.get_areas_with_simple_polygons([
             # We only index electoral wards in the RTree
             overlap.data for overlap in rtree_index.query(
-                Rect(*self.polygons.bounds)
+                Rect(*self.simple_polygons.bounds)
             )
         ])
 

app/models/broadcast_message.py

@@ -88,7 +88,7 @@ class BroadcastMessage(JSONModel):
             broadcast_message_id=broadcast_message_id,
         ))
 
-    @property
+    @cached_property
     def areas(self):
         if 'ids' in self._dict['areas']:
             library_areas = self.get_areas(self.area_ids)

tests/app/broadcast_areas/test_broadcast_area.py

@@ -276,24 +276,24 @@ def test_estimate_number_of_smartphones_for_population(
 @pytest.mark.parametrize('area, expected_phones_per_square_mile', (
     (
         # Islington (most dense in UK)
-        'lad20-E09000019', 34_281
+        'lad20-E09000019', 32_662
     ),
     (
         # Cordwainer Ward (City of London)
         # This is higher than Islington because we inflate the
         # popualtion to account for daytime workers
-        'wd20-E05009300', 496_480
+        'wd20-E05009300', 392_870
     ),
     (
         # Crewe East
-        'wd20-E05008621', 3_460),
+        'wd20-E05008621', 3_289),
     (
         # Eden (Cumbria, least dense in England)
-        'lad20-E07000030', 44.12
+        'lad20-E07000030', 43.41
     ),
     (
         # Highland (least dense in UK)
-        'lad20-S12000017', 8.18
+        'lad20-S12000017', 6.97
     ),
 ))
 def test_phone_density(
@@ -317,15 +317,15 @@ def test_phone_density(
     ),
     (
         # Crewe East
-        'wd20-E05008621', 1_476
+        'wd20-E05008621', 1_504
     ),
     (
         # Eden (Cumbria, least dense in England)
-        'lad20-E07000030', 3_844
+        'lad20-E07000030', 3_852
     ),
     (
         # Highland (least dense in UK)
-        'lad20-S12000017', 4_759
+        'lad20-S12000017', 4_846
     ),
     (
         # No population data available