Added support for querying on tags.

taggit/managers.py

 from django.contrib.contenttypes.models import ContentType
 from django.db import models
+from django.db.models.fields.related import ManyToManyRel
+from django.db.models.query_utils import QueryWrapper
 
 from taggit.models import Tag, TaggedItem
 from taggit.utils import require_instance_manager
 
 
+class TaggableRel(ManyToManyRel):
+    def __init__(self):
+        self.to = TaggedItem
+        self.related_name = None
+        self.limit_choices_to = {}
+        self.symmetrical = True
+        self.multiple = True
+        self.through = None
+
+
 class TaggableManager(object):
+    def __init__(self):
+        self.rel = TaggableRel()
+        self.editable = False
+        self.unique = False
+        self.creates_table = False
+        self.db_column = None
+    
     def __get__(self, instance, type):
         manager = _TaggableManager()
         manager.model = type
@@ -14,6 +33,43 @@ class TaggableManager(object):
         else:
             manager.object_id = instance.pk
         return manager
+    
+    def contribute_to_class(self, cls, name):
+        self.name = self.column = name
+        self.model = cls
+        cls._meta.add_field(self)
+        setattr(cls, name, self)
+
+    def save_form_data(self, instance, value):
+        pass
+    
+    def get_db_prep_lookup(self, lookup_type, value):
+        if lookup_type != "in":
+            raise ValueError("You can't do lookups other than in on Tags")
+        qs = TaggedItem.objects.filter(tag__name__in=value).values_list("pk", flat=True)
+        sql, params = qs.query.as_sql()
+        return QueryWrapper(("(%s)" % sql), params)
+    
+    def related_query_name(self):
+        return None
+    
+    def m2m_reverse_name(self):
+        return "id"
+    
+    def m2m_column_name(self):
+        return "object_id"
+    
+    def db_type(self):
+        return None
+    
+    def m2m_db_table(self):
+        return self.rel.to._meta.db_table
+    
+    def extra_filters(self, pieces, pos, negate):
+        if negate:
+            return []
+        prefix = "__".join(pieces[:pos+1])
+        return [("%s__content_type" % prefix, ContentType.objects.get_for_model(self.model))]
 
 
 class _TaggableManager(models.Manager):

taggit/models.py

@@ -18,4 +18,4 @@ class TaggedItem(models.Model):
     tag = models.ForeignKey(Tag, related_name="items")
     
     def __unicode__(self):
-        return "%s tagged with %" % (self.content_object, self.tag)
+        return "%s tagged with %s" % (self.content_object, self.tag)

taggit/tests/models.py

@@ -7,3 +7,12 @@ class Food(models.Model):
     name = models.CharField(max_length=50)
     
     tags = TaggableManager()
+    
+    def __unicode__(self):
+        return self.name
+
+
+class Pet(models.Model):
+    name = models.CharField(max_length=50)
+    
+    tags = TaggableManager()

taggit/tests/tests.py

@@ -2,13 +2,15 @@ from django.test import TestCase
 
 from taggit.models import Tag
 from taggit.tests.forms import FoodForm
-from taggit.tests.models import Food
+from taggit.tests.models import Food, Pet
+
 
 class BaseTaggingTest(TestCase):
     def assert_tags_equal(self, qs, tags):
         tags = Tag.objects.filter(name__in=tags)
         self.assertEqual(list(qs), list(tags))
 
+
 class AddTagTestCase(BaseTaggingTest):
     def test_add_tag(self):
         apple = Food.objects.create(name="apple")
@@ -29,11 +31,29 @@ class AddTagTestCase(BaseTaggingTest):
         self.assert_tags_equal(Food.tags.all(), ['green', 'red'])
         
         self.assert_tags_equal(Food.tags.most_common(), ['green', 'red'])
-        
+
         apple.tags.remove('green')
         self.assert_tags_equal(apple.tags.all(), ['red'])
         self.assert_tags_equal(Food.tags.all(), ['green', 'red'])
 
+
+class LookupByTagTestCase(BaseTaggingTest):
+    def test_lookup_by_tag(self):
+        apple = Food.objects.create(name="apple")
+        apple.tags.add("red", "green")
+        pear = Food.objects.create(name="pear")
+        pear.tags.add("green")
+        
+        self.assertEqual(list(Food.objects.filter(tags__in=["red"])), [apple])
+        self.assertEqual(list(Food.objects.filter(tags__in=["green"])), [apple, pear])
+        
+        kitty = Pet.objects.create(name="kitty")
+        kitty.tags.add("fuzzy", "red")
+        dog = Pet.objects.create(name="dog")
+        dog.tags.add("woof", "red")
+        self.assertEqual(list(Food.objects.filter(tags__in=["red"]).distinct()), [apple])
+
+
 class TaggableFormTestCase(BaseTaggingTest):
     def test_form(self):
         self.assertEqual(FoodForm.base_fields.keys(), ['name', 'tags'])