Using ActiveRecord, is there a way to get the old values of a record during after_update

Setup using a simple example: I've got 1 table (Totals) that holds the sum of the amount column of each record in a second table (Things).

When a thing.amount gets updated, I'd like to simply add the difference between the old value and the new value to total.sum.

Right now I'm subtracting self.amount during before_update and adding self.amount during after_update. This places WAY too much trust in the update succeeding.

Constraint: I don't want to simply recalculate the sum of all the transactions.

Question: Quite simply, I'd like to access the original value during an after_update callback. What ways have you come up with do this?

Update: I'm going with Luke Francl's idea. During an after_update callback you still have access to the self.attr_was values which is exactly what I wanted. I also decided to go with an after_update implementation because I want to keep this kind of logic in the model. This way, no matter how I decide to update transactions in the future, I'll know that I'm updating the sum of the transactions correctly. Thanks to everyone for your implementation suggestions.

Ditto what everyone is saying about transactions.

That said...

ActiveRecord as of Rails 2.1 keeps track of the attribute values of an object. So if you have an attribute total, you will have a total_changed? method and a total_was method that returns the old value.

There's no need to add anything to your model to keep track of this anymore.

Update: Here is the documentation for ActiveModel::Dirty as requested.

Appending "_was" to your attribute will give you the previous value before saving the data.

These methods are called dirty methods methods.

Cheers!

Some other folks are mentioning wrapping all this in a transaction, but I think that's done for you; you just need to trigger the rollback by raising an exception for errors in the after_* callbacks.

See http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html

The entire callback chain of a save, save!, or destroy call runs within a transaction. That includes after_* hooks. If everything goes fine a COMMIT is executed once the chain has been completed.

If a before_* callback cancels the action a ROLLBACK is issued. You can also trigger a ROLLBACK raising an exception in any of the callbacks, including after_* hooks. Note, however, that in that case the client needs to be aware of it because an ordinary save will raise such exception instead of quietly returning false.

To get all changed fields, with their old and new values respectively:

person = Person.create!(:name => 'Bill')
person.name = 'Bob'
person.save
person.changes        # => {"name" => ["Bill", "Bob"]}

ActiveRecord::Dirty is a module that's built into ActiveRecord for tracking attribute changes. So you can use thing.amount_was to get the old value.