Added support for prefetch_related on tags fields

taggit/managers.py

 from __future__ import unicode_literals
+from operator import attrgetter
 
 from django import VERSION
 from django.contrib.contenttypes.generic import GenericRelation
 from django.contrib.contenttypes.models import ContentType
-from django.db import models
+from django.db import models, router
 from django.db.models.fields import Field
 from django.db.models.fields.related import ManyToManyRel, RelatedField, add_lazy_relation
 from django.db.models.related import RelatedObject
@@ -82,7 +83,7 @@ class TaggableManager(RelatedField, Field):
             raise ValueError("%s objects need to have a primary key value "
                 "before you can access their tags." % model.__name__)
         manager = _TaggableManager(
-            through=self.through, model=model, instance=instance
+            through=self.through, model=model, instance=instance, prefetch_cache_name = self.name
         )
         return manager
 
@@ -314,14 +315,42 @@ class TaggableManager(RelatedField, Field):
 
 
 class _TaggableManager(models.Manager):
-    def __init__(self, through, model, instance):
+    def __init__(self, through, model, instance, prefetch_cache_name):
         self.through = through
         self.model = model
         self.instance = instance
+        self.prefetch_cache_name = prefetch_cache_name
+        self._db = None
+
+    def is_cached(self, instance):
+        return self.prefetch_cache_name in instance._prefetched_objects_cache
 
     def get_query_set(self):
+        try:
+            return self.instance._prefetched_objects_cache[self.prefetch_cache_name]
+        except (AttributeError, KeyError):
             return self.through.tags_for(self.model, self.instance)
 
+    def get_prefetch_query_set(self, instances):
+        instance = instances[0]
+        from django.db import connections
+        db = self._db or router.db_for_read(instance.__class__, instance=instance)
+
+        fk = self.through._meta.get_field('object_id' if issubclass(self.through, GenericTaggedItemBase) else 'content_object')
+        query = {
+            '%s__%s__in' % (self.through.tag_relname(), fk.name) : set(obj._get_pk_val() for obj in instances)
+        }
+        join_table = self.through._meta.db_table
+        source_col = fk.column
+        connection = connections[db]
+        qn = connection.ops.quote_name
+        qs = self.get_query_set().using(db)._next_is_sticky().filter(**query).extra(select = { '_prefetch_related_val' : '%s.%s' % (qn(join_table), qn(source_col))})
+        return (qs,
+                attrgetter('_prefetch_related_val'),
+                attrgetter(instance._meta.pk.name),
+                False,
+                self.prefetch_cache_name)
+
     # Django 1.6 renamed this
     get_queryset = get_query_set
 

tests/tests.py

@@ -341,6 +341,20 @@ class TaggableManagerTestCase(BaseTaggingTestCase):
         apple = self.food_model.objects.create(name="apple")
         serializers.serialize("json", (apple,))
 
+    def test_prefetch_related(self):
+        apple = self.food_model.objects.create(name="apple")
+        apple.tags.add('1', '2')
+        orange = self.food_model.objects.create(name="orange")
+        orange.tags.add('2', '4')
+        with self.assertNumQueries(2):
+            l = list(self.food_model.objects.prefetch_related('tags').all())
+        with self.assertNumQueries(0):
+            foods = {f.name : set(t.name for t in f.tags.all()) for f in l}
+            self.assertEqual(foods, {
+                u'orange': {'2', '4'},
+                u'apple': {'1', '2'}
+            })
+
 
 class TaggableManagerDirectTestCase(TaggableManagerTestCase):
     food_model = DirectFood