Save or Update Model


I have a model WatchedItem which is unique across two columns,
account_id and store_id.

id account_id store_id item_id
1 2 2 1

At the moment I have a method save_or_update which does a find on
account_id and store_id first. If this returns empty, a new row is
created, otherwise the row is updated. However as WatchedItems are
always saved in this way, I started thinking maybe I should extend the
save method for the model. What is the best rails/ruby practise for
encapusulating this?