Enable sort by recently tapped (#198)

* Add ability to sort beers by most recently added to a tap

This looks across *all* taps, so depending on how STA handles
changing kegs, this might make Monkeynaut always seem new.

* Validate that we don't add extra queries with the annotation

beers/filters.py

@@ -21,6 +21,7 @@ class BeerFilterSet(FilterSet):
         fields=[
             'name', 'abv', 'ibu', 'style__name',
             'style__alternate_names__name', 'manufacturer__name',
+            'most_recently_added',
         ],
     )
 

beers/test/test_views.py

@@ -1,3 +1,5 @@
+from dateutil.relativedelta import relativedelta
+from django.utils.timezone import now
 from django.urls import reverse
 from nose.tools import eq_
 from rest_framework.test import APITestCase
@@ -161,3 +163,32 @@ def test_on_tap_no_dupes(self):
         eq_(response.status_code, 200)
         eq_(len(response.data['results']), 1, response.data)
         eq_(response.data['results'][0]['name'], self.beer.name, response.data)
+
+    def test_sort_by_time_added(self):
+        # create a tap for a second beer in between the two
+        other_beer = BeerFactory()
+        TapFactory(
+            beer=other_beer,
+            time_added=now() - relativedelta(hours=5, minutes=2),
+        )
+        # create another beer on tap with no time added, so therefore it's now
+        third_beer = TapFactory(beer=BeerFactory()).beer
+
+        # create two taps for the first beer
+        TapFactory(beer=self.beer, time_added=now() - relativedelta(hours=1))
+        TapFactory(beer=self.beer, time_added=now() - relativedelta(hours=16))
+
+        # 5 queries:
+        # 1. count of results
+        # 2. beers
+        # 3. taps
+        # 4. alt names
+        # 5. prices
+        with self.assertNumQueries(5):
+            response = self.client.get(f'{self.url}?o=-most_recently_added')
+        eq_(response.status_code, 200)
+        # expected order is third_beer, self.beer, other_beer
+        eq_(len(response.data['results']), 3, response.data)
+        eq_(response.data['results'][0]['name'], third_beer.name, response.data)
+        eq_(response.data['results'][1]['name'], self.beer.name, response.data)
+        eq_(response.data['results'][2]['name'], other_beer.name, response.data)

beers/views.py

@@ -1,7 +1,7 @@
 
 from django.db import transaction
 from django.db.utils import IntegrityError
-from django.db.models import Prefetch, Count
+from django.db.models import Prefetch, Count, Max
 from django.http import HttpResponse
 from django.shortcuts import redirect
 from django.urls import reverse
@@ -73,6 +73,8 @@ class BeerViewSet(CachedListMixin, ModerationMixin, ModelViewSet):
                 'venue', 'serving_size',
             )
         ),
+    ).annotate(
+        most_recently_added=Max('taps__time_added'),
     ).order_by('manufacturer__name', 'name')
     filterset_class = filters.BeerFilterSet