Okay, now we are getting somewhere.

By itself, .instance_of(*models) will actually call select_subclasses(*models),
which results in just those objects being cast to subclasses.

However, if you want the casting to grandchildren (which is supported only in
django 1.6+), then you may use an extra call to .select_subclasses() (without
arguments, to select all subclasses).

It doesn't deal with @kezabelle's discussion of mixed-use of select_subclasses/
instance_of, I still need to look at that.

But it does pass all current tests...

model_utils/managers.py

@@ -171,6 +171,9 @@ class InheritanceQuerySetMixin(object):
         return levels
 
     def instance_of(self, *models):
+        """
+        Fetch only objects that are instances of the provided model(s).
+        """
         # If we aren't already selecting the subclasess, we need
         # to in order to get this to work.
         
@@ -190,7 +193,7 @@ class InheritanceQuerySetMixin(object):
                 ) for field in model._meta.parents.values()
             ]) + ')')
         
-        return self.extra(where=[' OR '.join(where_queries)])
+        return self.select_subclasses(*models).extra(where=[' OR '.join(where_queries)])
 
 class InheritanceManagerMixin(object):
     use_for_related_fields = True
@@ -206,8 +209,8 @@ class InheritanceManagerMixin(object):
     def get_subclass(self, *args, **kwargs):
         return self.get_queryset().get_subclass(*args, **kwargs)
 
-    def instance_of(self, model):
-        return self.get_queryset().instance_of(model)
+    def instance_of(self, *models):
+        return self.get_queryset().instance_of(*models)
 
 class InheritanceQuerySet(InheritanceQuerySetMixin, QuerySet):
     pass

model_utils/tests/tests.py

@@ -992,30 +992,51 @@ class InheritanceManagerUsingModelsTests(TestCase):
 
     def test_limit_to_specific_subclass(self):
         child3 = InheritanceManagerTestChild3.objects.create()
-        results = InheritanceManagerTestParent.objects.select_subclasses().instance_of(InheritanceManagerTestChild3)
+        results = InheritanceManagerTestParent.objects.instance_of(InheritanceManagerTestChild3)
         
         self.assertEqual([child3], list(results))
     
+    @skipUnless(django.VERSION >= (1, 6, 0), "test only applies to Django 1.6+")
     def test_limit_to_specific_grandchild_class(self):
         grandchild1 = InheritanceManagerTestGrandChild1.objects.get()
-        results = InheritanceManagerTestParent.objects.select_subclasses().instance_of(InheritanceManagerTestGrandChild1)
+        results = InheritanceManagerTestParent.objects.instance_of(InheritanceManagerTestGrandChild1)
 
         self.assertEqual([grandchild1], list(results))
     
-    def test_limit_to_child_fetches_grandchildren(self):
-        children = InheritanceManagerTestChild1.objects.select_subclasses().all()
+    def test_limit_to_child_fetches_grandchildren_as_child_class(self):
+        # Not sure if this is the desired behaviour...?
+        children = InheritanceManagerTestChild1.objects.all()
         
-        results = InheritanceManagerTestParent.objects.select_subclasses().instance_of(InheritanceManagerTestChild1)
+        results = InheritanceManagerTestParent.objects.instance_of(InheritanceManagerTestChild1)
+        
+        self.assertEqual(set(children), set(results))
+
+    @skipUnless(django.VERSION >= (1, 6, 0), "test only applies to Django 1.6+")
+    def test_can_fetch_limited_class_grandchildren(self):
+        # Not sure if this is the desired behaviour...?
+        children = InheritanceManagerTestChild1.objects.select_subclasses()
+        
+        results = InheritanceManagerTestParent.objects.instance_of(InheritanceManagerTestChild1).select_subclasses()
         
         self.assertEqual(set(children), set(results))
     
     def test_selecting_multiple_instance_classes(self):
+        child3 = InheritanceManagerTestChild3.objects.create()
+        children1 = InheritanceManagerTestChild1.objects.all()
+        
+        results = InheritanceManagerTestParent.objects.instance_of(InheritanceManagerTestChild3, InheritanceManagerTestChild1)
+        
+        self.assertEqual(set([child3] + list(children1)), set(results))
+    
+    @skipUnless(django.VERSION >= (1, 6, 0), "test only applies to Django 1.6+")
+    def test_selecting_multiple_instance_classes_including_grandchildren(self):
         child3 = InheritanceManagerTestChild3.objects.create()
         grandchild1 = InheritanceManagerTestGrandChild1.objects.get()
         
-        results = InheritanceManagerTestParent.objects.select_subclasses().instance_of(InheritanceManagerTestChild3, InheritanceManagerTestGrandChild1)
+        results = InheritanceManagerTestParent.objects.instance_of(InheritanceManagerTestChild3, InheritanceManagerTestGrandChild1).select_subclasses()
         
         self.assertEqual(set([child3, grandchild1]), set(results))
+        
 
 class InheritanceManagerRelatedTests(InheritanceManagerTests):
     def setUp(self):