til/rails/handle-bad-numerical-amounts-with-big-decimal.md

Handle Bad Numerical Amounts With BigDecimal

I'm working on a payment page with a backing Rails controller. The user can select between their full balance or some partial payment amount. Because this form accepts an arbitrary value for the amount, I need to do some server-side validation.

While I could parse the amount value and handle the exception that gets raised on bad numerical values, I'd rather have bad values coerce to nil and let downstream validations handle it from there.

BigDecimal can help here with the support of its exception option.

> BigDecimal('123', exception: false)
=> 0.123e3
> BigDecimal('taco', exception: false)
=> nil

Maybe that parsing logic ends up looking something like this:

def parse_payment_amount(value, current_balance)
  amount = BigDecimal(value, exception: false)

  if amount.present? && amount == current_balance
    [:full_balance, amount]
  else
    [:other, amount]
  end
end

I return tuples where you either get a verified :full_balance amount or you get some :other amount. The other amount could be nil which would trigger a downstream validation.