diff --git a/activerecord/lib/active_record/attribute_methods/primary_key.rb b/activerecord/lib/active_record/attribute_methods/primary_key.rb
index 93dae3ff868cb32aa5d4bb22c880c530ecda1f16..a5c2849844bc067e2094362196eb983bc527f845 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -5,7 +5,7 @@ module PrimaryKey
 
       # Returns this record's primary key value wrapped in an Array if one is available
       def to_key
-        key = send(self.class.primary_key)
+        key = self.id
         [key] if key
       end