Added no_sub_classes context manager and queryset helper (#312)

tests/queryset/queryset.py

@@ -3322,7 +3322,7 @@ class QuerySetTest(unittest.TestCase):
 
             self.assertEqual(q, 2)
 
-    def test_disable_inheritance_queryset(self):
+    def test_no_sub_classes(self):
         class A(Document):
             x = IntField()
             y = IntField()
@@ -3332,15 +3332,34 @@ class QuerySetTest(unittest.TestCase):
         class B(A):
             z = IntField()
 
+        class C(B):
+            zz = IntField()
+
         A.drop_collection()
 
         A(x=10, y=20).save()
         A(x=15, y=30).save()
         B(x=20, y=40).save()
         B(x=30, y=50).save()
+        C(x=40, y=60).save()
 
-        for obj in A.objects.disable_inheritance():
+        self.assertEqual(A.objects.no_sub_classes().count(), 2)
+        self.assertEqual(A.objects.count(), 5)
+
+        self.assertEqual(B.objects.no_sub_classes().count(), 2)
+        self.assertEqual(B.objects.count(), 3)
+
+        self.assertEqual(C.objects.no_sub_classes().count(), 1)
+        self.assertEqual(C.objects.count(), 1)
+
+        for obj in A.objects.no_sub_classes():
             self.assertEqual(obj.__class__, A)
 
+        for obj in B.objects.no_sub_classes():
+            self.assertEqual(obj.__class__, B)
+
+        for obj in C.objects.no_sub_classes():
+            self.assertEqual(obj.__class__, C)
+
 if __name__ == '__main__':
     unittest.main()