1 files changed, 16 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/attribute_methods/primary_key.rb b/activerecord/lib/active_record/attribute_methods/primary_key.rb
index 365fdeb55a..095814b635 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -39,6 +39,22 @@ module ActiveRecord
         end
         alias :primary_key= :set_primary_key
       end
+
+      module InstanceMethods
+
+        # Returns this record's primary key value wrapped in an Array
+        # or nil if the record is a new_record?
+        # This is done to comply with the AMo interface that expects
+        # every AMo compliant object to respond_to?(:to_key) and return
+        # an Enumerable object from that call, or nil if new_record?
+        # This method also takes custom primary keys specified via
+        # the +set_primary_key+ into account.
+        def to_key
+          new_record? ? nil : [ self.primary_key ]
+        end
+
+      end
+
     end
   end
 end