title: Using ActiveRecord, is there a way to get the old values of a record during after_update
body: <p><strong>Setup using a simple example:</strong> I've got 1 table (<code>Totals</code>) that holds the sum of the <code>amount</code> column of each record in a second table (<code>Things</code>).</p>
  
  <p>When a <code>thing.amount</code> gets updated, I'd like to simply add the difference between the old value and the new value to <code>total.sum</code>.</p>
  
  <p>Right now I'm subtracting <code>self.amount</code> during <code>before_update</code> and adding <code>self.amount</code> during <code>after_update</code>.  This places WAY too much trust in the update succeeding.  </p>
  
  <p><strong>Constraint:</strong>  I don't want to simply recalculate the sum of all the transactions.</p>
  
  <p><strong>Question:</strong> Quite simply, I'd like to access the original value during an <code>after_update</code> callback.  What ways have you come up with do this?</p>
  
  <p><strong>Update:</strong> I'm going with Luke Francl's idea.  During an <code>after_update</code> callback you still have access to the <code>self.attr_was</code> values which is exactly what I wanted.  I also decided to go with an <code>after_update</code> 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.</p>