diff --git a/channels/binding/base.py b/channels/binding/base.py
index 902b56e..a1f1977 100644
--- a/channels/binding/base.py
+++ b/channels/binding/base.py
@@ -140,7 +140,7 @@ class Binding(object):
         if action == CREATE:
             group_names = set()
         else:
-            group_names = set(cls.group_names(instance))
+            group_names = set(cls.group_names(instance, action))
 
         if not hasattr(instance, '_binding_group_names'):
             instance._binding_group_names = {}
@@ -155,7 +155,7 @@ class Binding(object):
         if action == DELETE:
             new_group_names = set()
         else:
-            new_group_names = set(cls.group_names(instance))
+            new_group_names = set(cls.group_names(instance, action))
 
         # if post delete, new_group_names should be []
         self = cls()
@@ -182,7 +182,8 @@ class Binding(object):
             group = Group(group_name)
             group.send(message)
 
-    def group_names(self, instance, action):
+    @classmethod
+    def group_names(cls, instance, action):
         """
         Returns the iterable of group names to send the object to based on the
         instance and action performed on it.
diff --git a/channels/tests/test_binding.py b/channels/tests/test_binding.py
index dca1092..aaf88aa 100644
--- a/channels/tests/test_binding.py
+++ b/channels/tests/test_binding.py
@@ -18,7 +18,7 @@ class TestsBinding(ChannelTestCase):
             fields = ['username', 'email', 'password', 'last_name']
 
             @classmethod
-            def group_names(cls, instance):
+            def group_names(cls, instance, action):
                 return ["users"]
 
             def has_permission(self, user, action, pk):
@@ -60,7 +60,7 @@ class TestsBinding(ChannelTestCase):
             fields = ['__all__']
 
             @classmethod
-            def group_names(cls, instance):
+            def group_names(cls, instance, action):
                 return ["users2"]
 
             def has_permission(self, user, action, pk):
@@ -105,7 +105,7 @@ class TestsBinding(ChannelTestCase):
             fields = ['username']
 
             @classmethod
-            def group_names(cls, instance):
+            def group_names(cls, instance, action):
                 return ["users3"]
 
             def has_permission(self, user, action, pk):