Optimize the query in `bulk_lookup_kwargs` for QuerySet args.

When passing a QuerySet to bulk_lookup_kwargs(), the queryset was
undesirably executed to get a list of IDs. This causes an unnecessary
performance loss.

There is no need to loop over a QuerySet to execute a IN (1, 3, 4) query.
Instead, just allow the ORM and database to execute a IN (SELECT id FROM ..) query.
In a real life app with a large set of objects, the difference in performance is noticeable.

taggit/models.py

@@ -2,6 +2,7 @@ import django
 from django.contrib.contenttypes.models import ContentType
 from django.contrib.contenttypes.generic import GenericForeignKey
 from django.db import models, IntegrityError, transaction
+from django.db.models.query import QuerySet
 from django.template.defaultfilters import slugify as default_slugify
 from django.utils.translation import ugettext_lazy as _, ugettext
 
@@ -137,11 +138,18 @@ class GenericTaggedItemBase(ItemBase):
 
     @classmethod
     def bulk_lookup_kwargs(cls, instances):
-        # TODO: instances[0], can we assume there are instances.
-        return {
-            "object_id__in": [instance.pk for instance in instances],
-            "content_type": ContentType.objects.get_for_model(instances[0]),
-        }
+        if isinstance(instances, QuerySet):
+            # Can do a real object_id IN (SELECT ..) query.
+            return {
+                "object_id__in": instances,
+                "content_type": ContentType.objects.get_for_model(instances.model),
+            }
+        else:
+            # TODO: instances[0], can we assume there are instances.
+            return {
+                "object_id__in": [instance.pk for instance in instances],
+                "content_type": ContentType.objects.get_for_model(instances[0]),
+            }
 
     @classmethod
     def tags_for(cls, model, instance=None):

taggit/tests/tests.py

@@ -215,6 +215,27 @@ class TaggableManagerTestCase(BaseTaggingTestCase):
             [kitty.pk, cat.pk]
         )
 
+    def test_lookup_bulk(self):
+        apple = self.food_model.objects.create(name="apple")
+        pear = self.food_model.objects.create(name="pear")
+        apple.tags.add('fruit', 'green')
+        pear.tags.add('fruit', 'yummie')
+
+        def lookup_qs():
+            # New fix: directly allow WHERE object_id IN (SELECT id FROM ..)
+            objects = self.food_model.objects.all()
+            lookup = self.taggeditem_model.bulk_lookup_kwargs(objects)
+            list(self.taggeditem_model.objects.filter(**lookup))
+
+        def lookup_list():
+            # Simulate old situation: iterate over a list.
+            objects = list(self.food_model.objects.all())
+            lookup = self.taggeditem_model.bulk_lookup_kwargs(objects)
+            list(self.taggeditem_model.objects.filter(**lookup))
+
+        self.assert_num_queries(1, lookup_qs)
+        self.assert_num_queries(2, lookup_list)
+
     def test_exclude(self):
         apple = self.food_model.objects.create(name="apple")
         apple.tags.add("red", "green", "delicious")