Summing `Money`s

We ran into an interesting bug today. We had code that looked like this:

foos.sum(&:amount) == expected_total

where foos is a collection that has an amount property that returns a Money, and expected_total is also a Money.

This works fine, except when foos is empty. In that case, the default value 0 (an Integer) is used, and as it turns out:

0 == Money.new(0, "USD") # => false

The fix is easy enough, just provide an explicit initial value:

foos.sum(Money.new(0, "USD"), &:amount) == expected_total

but this still seems like a very easy way for developers to shoot themselves in the foot when dealing with Money. I was wondering if Shopify (or others) have dealt with this issue before and have any recommendations for avoiding this issue.

One thought I had was a Rubocop rule that requires an explicit initial value for all #sum calls, but that seems overkill and annoying since it works fine when dealing with non-Money numerics.

Start with the reported Ruby Enumerable#sum behavior for an empty collection whose values are Money, using the examples in this issue. No repository file or test is named; first locate the relevant sum integration and existing Money tests, then define a regression test and confirm the project agrees on the desired behavior.