Add support for a 'country' filter in query parameters

This change means that you can add '?country=E' (say) to queries for
multiple areas to restrict the results to those areas that have a
Country with code 'E'.

This should be particularly helpful for global.mapit.mysociety.org
if we set an appropriate Country for each Area, since one is almost
always interested in results from a particular country.

Fixes #55

mapit/tests/test_query_args.py

@@ -108,3 +108,25 @@ def test_min_generation_specified(self):
                 'generation_low__lte': self.active_generation.id,
             }
         )
+
+    def test_one_country_in_query(self):
+        args = query_args(FakeRequest({'country': 'DE'}), 'json', None)
+        self.assertEqual(
+            args,
+            {
+                'generation_high__gte': self.active_generation.id,
+                'generation_low__lte': self.active_generation.id,
+                'country__code': 'DE',
+            }
+        )
+
+    def test_two_countries_in_query(self):
+        args = query_args(FakeRequest({'country': 'DE,FR'}), 'json', None)
+        self.assertEqual(
+            args,
+            {
+                'generation_high__gte': self.active_generation.id,
+                'generation_low__lte': self.active_generation.id,
+                'country__code__in': ['DE', 'FR'],
+            }
+        )

mapit/views/areas.py

@@ -64,17 +64,22 @@ def query_args(request, format, type=None):
 
     if type is None:
         type = request.GET.get('type', '')
+    country_code = request.GET.get('country', '')
 
     args = {}
     if min_generation > -1:
         args = {
             'generation_low__lte': generation,
             'generation_high__gte': min_generation,
         }
-    if ',' in type:
-        args['type__code__in'] = type.split(',')
-    elif type:
-        args['type__code'] = type
+    for attr, value in [
+            ('type', type),
+            ('country', country_code),
+    ]:
+        if ',' in value:
+            args[attr + '__code__in'] = value.split(',')
+        elif value:
+            args[attr + '__code'] = value
 
     return args