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39 lines
1.2 KiB
Markdown
39 lines
1.2 KiB
Markdown
# Handle Bad Numerical Amounts With BigDecimal
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I'm working on a payment page with a backing Rails controller. The user can
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select between their full balance or some partial payment amount. Because this
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form accepts an arbitrary value for the amount, I need to do some server-side
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validation.
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While I could parse the `amount` value and handle the exception that gets raised
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on bad numerical values, I'd rather have bad values coerce to `nil` and let
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downstream validations handle it from there.
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[`BigDecimal`](https://docs.ruby-lang.org/en/master/BigDecimal.html) can help
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here with the support of its `exception` option.
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```ruby
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> BigDecimal('123', exception: false)
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=> 0.123e3
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> BigDecimal('taco', exception: false)
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=> nil
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```
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Maybe that parsing logic ends up looking something like this:
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```ruby
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def parse_payment_amount(value, current_balance)
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amount = BigDecimal(value, exception: false)
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if amount.present? && amount == current_balance
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[:full_balance, amount]
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else
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[:other, amount]
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end
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end
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```
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I return tuples where you either get a verified `:full_balance` amount or you
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get some `:other` amount. The other amount could be `nil` which would trigger a
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downstream validation.
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