Improve collections ajax list performance. (bug 1211692)

This uses a simple extra query to check if the addon
exists in a collection or not.

This avoids the for-loop and thus n queries on a (possibly) huge
list of addon ids.

apps/bandwagon/models.py

@@ -45,10 +45,29 @@ def __set__(self, obj, value):
         cache.set(self.key(obj), value, two_days)
 
 
+class CollectionQuerySet(caching.CachingQuerySet):
+
+    def with_has_addon(self, addon_id):
+        """Add a `has_addon` property to each collection.
+
+        `has_addon` will be `True` if `addon_id` exists in that
+        particular collection.
+        """
+        has_addon = """
+            select 1 from addons_collections as ac
+                where ac.addon_id = %s and ac.collection_id = collections.id
+                limit 1"""
+
+        return self.extra(
+            select={'has_addon': has_addon},
+            select_params=(addon_id,))
+
+
 class CollectionManager(amo.models.ManagerBase):
 
     def get_query_set(self):
         qs = super(CollectionManager, self).get_query_set()
+        qs = qs._clone(klass=CollectionQuerySet)
         return qs.transform(Collection.transformer)
 
     def manual(self):

apps/bandwagon/tests/test_models.py

@@ -214,6 +214,26 @@ def test_can_view_stats(self):
         eq_(c.can_view_stats(fake_request), True)
 
 
+class TestCollectionQuerySet(amo.tests.TestCase):
+    fixtures = ('base/addon_3615',)
+
+    def test_with_has_addon(self):
+        user = UserProfile.objects.create(username='uhhh', email='uh@hh')
+        collection = Collection.objects.create(author=user)
+        addon = Addon.objects.all()[0]
+
+        qset = (
+            Collection.objects
+            .filter(pk=collection.id)
+            .with_has_addon(addon.id))
+
+        assert not qset.first().has_addon
+
+        collection.add_addon(addon)
+
+        assert qset.first().has_addon
+
+
 class TestRecommendations(amo.tests.TestCase):
     fixtures = ['base/addon-recs']
     ids = [5299, 1843, 2464, 7661, 5369]

apps/bandwagon/views.py

@@ -369,13 +369,10 @@ def ajax_list(request):
     except (KeyError, ValueError):
         return http.HttpResponseBadRequest()
 
-    # Get collections associated with this user
-    collections = Collection.objects.publishable_by(request.amo_user)
-
-    for collection in collections:
-        # See if the collections contains the addon
-        if addon_id in collection.addons.values_list('id', flat=True):
-            collection.has_addon = True
+    collections = (
+        Collection.objects
+        .publishable_by(request.amo_user)
+        .with_has_addon(addon_id))
 
     return render(request, 'bandwagon/ajax_list.html',
                   {'collections': collections})