An improved backwards migration for reverting from 7-area-unions to m…

…aster

The previous version of the backwards part of migration 0006 could
lose geometries.  This version preserves all geometries that were
associated with a area, and unions them to minimize the number of
geometries required to represent the area.

Thanks to Jenny Duckett for helpful suggestions on how to rewrite
this so that it's shorter and clearer.

mapit/migrations/0006_move_geometry_area_ids.py

@@ -5,18 +5,48 @@
 from django.db import models
 from django.conf import settings
 
+from django.contrib.gis.geos import MultiPolygon
+
 class Migration(DataMigration):
 
     def forwards(self, orm):
         for g in orm.Geometry.objects.all().iterator():
             g.areas.add( g.area )
             g.save()
 
-
     def backwards(self, orm):
-        for g in orm.Geometry.objects.all().iterator():
-            g.area = g.areas.all()[0]
-            g.save()
+        # Going backwards, we're moving from the situation where a
+        # geometry can be in multiple areas, to only being in a single
+        # area.  Please note that this isn't guaranteed to recreate
+        # exactly the same areas and geometries after going forwards
+        # and backwards through this migration, but for most purposes
+        # it'll be functionally the same.
+        for a in orm.Area.objects.all():
+            # Find any Geometry where area was set to this area, and
+            # set its area to NULL.  This won't be necessary if you've
+            # just migrated backwards through 0007, but there might be
+            # some if you've just migrated forward to 0006:
+            a.polygons.clear()
+            # We want to duplicate every geometry associated with the
+            # area, and set this area on it - we duplicate them
+            # because any given geometry might be associated with
+            # other areas too.
+            polygons_to_duplicate = [g.polygon for g in a.geometries.all()]
+            # However, it's quite possible that these geometries are
+            # adjacent, so do a cascaded union of them to simplify the
+            # number of polygons used.  (Unfortunately, we can't just
+            # do unionagg on the query set, because with South's
+            # unfrozen models it's a QuerySet, not a GeoQuerySet.)
+            mp = MultiPolygon(polygons_to_duplicate)
+            unioned = mp.cascaded_union
+            if unioned.geom_type == 'Polygon':
+                unioned = [unioned]
+            for polygon in unioned:
+                a.polygons.create(polygon=polygon)
+        # Now remove any geometries that have area_id set to NULL -
+        # these will be those only associated with areas via the old
+        # join table.
+        orm.Geometry.objects.filter(area=None).delete()
 
     models = {
         'mapit.area': {