Check that all areas are in the library

We should assume to start with that areas come either from the library
or from the JSON, and not a combination of the two.

app/models/broadcast_message.py

@@ -78,9 +78,17 @@ class BroadcastMessage(JSONModel):
 
     @property
     def areas(self):
-        return self.get_areas(
-            areas=self._dict['areas']
-        ) or CustomBroadcastAreas(
+        library_areas = self.get_areas(areas=self._dict['areas'])
+
+        if library_areas:
+            if len(library_areas) != len(self._dict['areas']):
+                raise RuntimeError(
+                    f'BroadcastMessage has {len(self._dict["areas"])} areas '
+                    f'but {len(library_areas)} found in the library'
+                )
+            return library_areas
+
+        return CustomBroadcastAreas(
             areas=self._dict['areas'],
             polygons=self._dict['simple_polygons'],
         )

tests/app/models/test_broadcast_message.py

@@ -1,3 +1,5 @@
+import pytest
+
 from app.models.broadcast_message import BroadcastMessage
 from tests import broadcast_message_json
 
@@ -45,3 +47,24 @@ def test_content_comes_from_attribute_not_template(fake_uuid):
         created_by_id=fake_uuid,
     ))
     assert broadcast_message.content == 'This is a test'
+
+
+def test_raises_for_missing_areas(fake_uuid):
+    broadcast_message = BroadcastMessage(broadcast_message_json(
+        id_=fake_uuid,
+        service_id=fake_uuid,
+        template_id=fake_uuid,
+        status='draft',
+        created_by_id=fake_uuid,
+        areas=[
+            'wd20-E05009372',
+            'something else',
+        ],
+    ))
+
+    with pytest.raises(RuntimeError) as exception:
+        broadcast_message.areas
+
+    assert str(exception.value) == (
+        'BroadcastMessage has 2 areas but 1 found in the library'
+    )