Freezing associated objects

Question

Does anyone know of any method in Rails by which an associated object may be frozen. The problem I am having is that I have an order model with many line items which in turn belong to a product or service. When the order is paid for, I need to freeze the details of the ordered items so that when the price is changed, the order's totals are preserved.

Answer 1

I worked on an online purchase system before. What you want to do is have an Order class and a LineItem class. LineItems store product details like price, quantity, and maybe some other information you need to keep for records. It's more complicated but it's the only way I know to lock in the details.

An Order is simply made up of LineItems and probably contains shipping and billing addresses. The total price of the Order can be calculated by adding up the LineItems.

Basically, you freeze the data before the person makes the purchase. When they are added to an order, the data is frozen because LineItems duplicate nessacary product information. This way when a product is removed from your system, you can still make sense of old orders.

You may want to look at a rails plugin call 'AASM' (formerly, acts as state machine) to handle the state of an order.

Edit: AASM can be found here http://github.com/rubyist/aasm/tree/master

Answer 2

Shouldn't the prices already be frozen when the items are added to the order? Say I put a widget into my shopping basket thinking it costs $1 and by the time I'm at the register, it costs $5 because you changed the price.

Back to your problem: I don't think it's a language issue, but a functional one. Instead of associating the prices with items, you need to copy the prices. If every item in the order has it's own version of a price, future price changes won't effect it, you can add discounts, etc.

Actually, to be clean you need to add versioning to your prices. When an item's price changes, you don't overwrite the price, you add a newer version. The line items in your order will still be associated with the old price.

Answer 3

A few options:

1) Add a version number to your model. At the day job we do course scheduling. A particular course might be updated occasionally but, for business rule reasons, its important to know what it looked like on the day you signed up. Add :version_number to model and find_latest_course(course_id), alter code as appropriate, stir a bit. In this case you don't "edit" models so much as you do a new save of the new, updated version. (Then, obviously, your LineItems carry a item_id and an item_version_number.)

This generic pattern can be extended to cover, shudder, audit trails.

2) Copy data into LineItem objects at LineItem creation time. Just because you can slap has_a on anything, doesn't mean you should. If a 'LineItem' is supposed to hold a constant record of one item which appeared on an invoice, then make the LineItem hold a constant record of one item which appeared on an invoice. You can then update InventoryItem#current_price at will without affecting your previously saved LineItems.

3) If you're lazy, just freeze the price on the order object. Not really much to recommend this but, hey, it works in a pinch. You're probably just delaying the day of reckoning though.

"I ordered from you 6 months ago and now am doing my taxes. Why won't your bookstore show me half of the books I ordered? What do you mean their IDs were purged when you stopped selling them?! I need to know which I can get deductions for!"