Possible miscalculation of discounts in transactions

[jkelleyj]

In transactions, the "weightage" function used to distribute the discounts across line items is a little off. It uses the unit_price instead of the unit_price * quantity https://github.com/vinsol-spree-contrib/spree_taxjar/blob/master/app/models/spree/taxjar.rb#L207

I believe this will under-represents the discounts, though this may have no bearing on the tax reported if TaxJar uses the amount field exclusively.

Have you experienced any API validation errors regarding the discounts not matching the totals?

[avneetmalhotra]

Hi @jkelleyj

Thanks for reporting this issue. We will fix it soon.

[jkelleyj]

Thanks @avneetmalhotra

I've been working with the extension and making some changes in my own repo, so I haven't fully confirmed this issue. According to TaxJar's api, the discounts on the line item should be the full discount amount for that line (not the per unit discount):
https://developers.taxjar.com/api/reference/#post-create-an-order-transaction

It says the API will raise a 400 error if the amount and the line item amounts + discounts don't line up, so I'm surprised no one using this extension has seen that error if this is actually a bug. Will let you know as I find more details in our testing.

[jkelleyj]

I'm also a little suspicious of using @order.item_total as the denominator in that weightage calculation since this is defined as sum(price*quantity) and does not factor in line item discounts as taxable_amount would.

Maybe a better approach is: weightage = @order.adjustments.sum(:amount) / @order.line_items.map(&:taxable_amount).sum

This weightage could be multiplied by the taxable amount of the line item so we get a pure ratio that will be guaranteed to add up to 100% of the adjustments sum