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

Question

Hey everyone,

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 to 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.

Answer 1

Firstly, you should be doing this in a transaction to ensure that your data gets written together.

To answer your question, you could just set a member variable to the old value in the before_update, which you can then access in the after_update, however this isn't a very elegant solution.

Answer 2

Idea 1: Wrap the update in a database transaction, so that if the update fails your Totals table isn't changed: ActiveRecord Transactions docs

Idea 2: Stash the old value in @old_total during the before_update.

Answer 3

Add this to your model:

def amount=(new_value)
    @old_amount = read_attribute(:amount)
    write_attribute(:amount,new_value)
end

Then use @old_amount in your after_update code.

Answer 4

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 any more.

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

Answer 5

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.

Answer 6

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.