WIP: this fails on django < 1.6

I think this is actually related to the issue related in #23.

model_utils/managers.py

@@ -170,7 +170,22 @@ class InheritanceQuerySetMixin(object):
             levels = 1
         return levels
 
-    def instance_of(self, model):
+    def instance_of(self, *models):
+        # If we aren't already selecting the subclasess, we need
+        # to in order to get this to work.
+        
+        where_queries = []
+        for model in models:
+            where_queries.append('(' + ' AND '.join([
+                '"%s"."%s" IS NOT NULL' % (
+                    model._meta.db_table,
+                    field.attname
+                ) for field in model._meta.parents.values()
+            ]) + ')')
+        
+        return self.extra(where=[' OR '.join(where_queries)])
+        
+        # We need to get the 
         parent_field = model._meta.parents.values()[0].attname
         query = '"%s"."%s" IS NOT NULL' % (
             model._meta.db_table, 

model_utils/tests/tests.py

@@ -994,9 +994,28 @@ class InheritanceManagerUsingModelsTests(TestCase):
         child3 = InheritanceManagerTestChild3.objects.create()
         results = InheritanceManagerTestParent.objects.select_subclasses().instance_of(InheritanceManagerTestChild3)
         
-        self.assertEqual(1, len(results))
         self.assertEqual([child3], list(results))
+    
+    def test_limit_to_specific_grandchild_class(self):
+        grandchild1 = InheritanceManagerTestGrandChild1.objects.get()
+        results = InheritanceManagerTestParent.objects.select_subclasses().instance_of(InheritanceManagerTestGrandChild1)
 
+        self.assertEqual([grandchild1], list(results))
+    
+    def test_limit_to_child_fetches_grandchildren(self):
+        children = InheritanceManagerTestChild1.objects.select_subclasses().all()
+        
+        results = InheritanceManagerTestParent.objects.select_subclasses().instance_of(InheritanceManagerTestChild1)
+        
+        self.assertEqual(set(children), set(results))
+    
+    def test_selecting_multiple_instance_classes(self):
+        child3 = InheritanceManagerTestChild3.objects.create()
+        grandchild1 = InheritanceManagerTestGrandChild1.objects.get()
+        
+        results = InheritanceManagerTestParent.objects.select_subclasses().instance_of(InheritanceManagerTestChild3, InheritanceManagerTestGrandChild1)
+        
+        self.assertEqual(set([child3, grandchild1]), set(results))
 
 class InheritanceManagerRelatedTests(InheritanceManagerTests):
     def setUp(self):