Remove spree_adjustments, replace with {order,line_item,shipment}_{cancellation,discounts,taxes}

[mamhoff]

Adjustments are one of the hardest-to-parse models in our code base because they're polymorphic in two directions. The polymorphic nature of the model/table also makes it impossible (or pretty hard) to set good constraints on the database level.

Furthermore, adjustments have to be treated differently when they're discounts or taxes: Discounts have to applied before taxes.

The codebase is pretty full of type checks regarding adjustments:

• https://github.com/solidusio/solidus/blob/master/core/app/models/spree/adjustment.rb#L34-L38
• https://github.com/solidusio/solidus/blob/master/core/app/models/spree/adjustment.rb#L109
• https://github.com/solidusio/solidus/blob/master/core/app/models/spree/adjustment.rb#L122
• https://github.com/solidusio/solidus/blob/master/core/app/models/spree/order_updater.rb#L200

I think it would be a huge improvement for the readability of the code base. The migration would not be hard to write, even in pure SQL.

What do y'all think?

[tvdeyen]

I support that. It would be much easier for people unfamiliar with the code base and would make code better to unterstand and to maintain.

IMO adjustments should stay solely as a manual added change to the order total. That's what we already have an interface and features for (Adjustment Reasons).

Would love to hear what others think.

[jarednorman]

It's an interesting idea, certainly. My main concern here is that this would create quite the break in compatibility.

[tvdeyen]

The idea is to

a) Add a feature toggle that is false by default
b) Implement the code and act on the toggle accordingly
c) Ship a version of Solidus that introduces the system, but turned off by default
d) Ship a major version where the new system is turned on by default

But, yes. This is a major change and lot's of work.

[waiting-for-dev]

It makes a lot of sense. As you said, it's a lot of work (for sure, more than we can foresee), but if somebody is willing to take it, we can try!

[tvdeyen]

"somebody" is already working on it #4296 ;)