I’m a pretty big fan of Ruby on Rails and occasionally I come across some particularly well-made parts. Rails’ ability to track changes in objects in memory has been around well over a year but its staggering how much thought goes into what people might want.

How do I check the value of an object before it was changed?

Let’s say you want to keep track of the total amount a user has across all orders. Rails has built-in not only a convenient place to update the value, but it has the previous value so you can find out the difference.

So nice!

In the example below,

• there is a field in the order table called ‘order_total’.
• order_total_change is Rails-generated array returning e.g. [20.53, 21.52] if say, the order_total had gone up by 1.99.
• order_total_changed? is also Rails-autogenerated.

class Order
...
before_update :update_total_user_spend
...
def update_total_user_spend
  if order_total_changed?
    previous_value, new_value = self.order_total_change
    self.user.total_spend += new_value - previous_value
    self.user.save
  end
end

Rock on!

(yes and I do need to adjust this blog template so it has a little more room for CONTENT