Spree::Money should be comparable

[eoinkelly]

Is there a reason why Spree::Money does not implement <=> and include Comparable? The current implementation manually implements #== but not <=>. I think it would be convenient to be able to sort Spree::Money instances directly rather than having to reach into their wrapped Money instances.

I'm happy to have a go at a PR for this. Is there any constraint/reason I'm not seeing that would make it a bad idea?

[mamhoff]

We'd like Spree::Money to implement all the things that the base Money object supports, one by one. If this particular itches you, by all means submit a PR!

[kennyadsl]

@eoinkelly How do you want to implement this? I think we only should allow comparing Money objects that have the same currency with <=>.

[jhawthorn]

@kennyadsl I agree. That seems to be how ::Money works already, so delegating to that will have the desired behaviour.

> ::Money.new(10) < ::Money.new(20)
=> true
> ::Money.new(10) > ::Money.new(20)
=> false
> ::Money.new(10, "USD") < ::Money.new(20, "CAD")
ArgumentError: comparison of Money with Money failed

@eoinkelly A PR would be most welcome

[eoinkelly]

Not comparing across currencies seems sensible. I'll have a go at a PR and then we'll have something concrete to discuss.