From f83b5003de8a87f71e1e342888390a8330288de0 Mon Sep 17 00:00:00 2001
From: jbranchaud <jbranchaud@gmail.com>
Date: Wed, 12 Aug 2026 08:32:38 -0500
Subject: [PATCH] Add Handle Bad Numerical Amounts With BigDecimal as a Rails
 TIL

---
 README.md                                     |  3 +-
 ...-bad-numerical-amounts-with-big-decimal.md | 38 +++++++++++++++++++
 2 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 rails/handle-bad-numerical-amounts-with-big-decimal.md

diff --git a/README.md b/README.md
index 34b9335..d7efd7c 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/).
 
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+_1860 TILs and counting..._
 
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@@ -1223,6 +1223,7 @@ If you've learned something here, support my efforts writing daily TILs by
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+- [Handle Bad Numerical Amounts With BigDecimal](rails/handle-bad-numerical-amounts-with-big-decimal.md)
 - [Handle Named Arguments In A Rake Task](rails/handle-named-arguments-in-a-rake-task.md)
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diff --git a/rails/handle-bad-numerical-amounts-with-big-decimal.md b/rails/handle-bad-numerical-amounts-with-big-decimal.md
new file mode 100644
index 0000000..9b319af
--- /dev/null
+++ b/rails/handle-bad-numerical-amounts-with-big-decimal.md
@@ -0,0 +1,38 @@
+# 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`](https://docs.ruby-lang.org/en/master/BigDecimal.html) can help
+here with the support of its `exception` option.
+
+```ruby
+> BigDecimal('123', exception: false)
+=> 0.123e3
+> BigDecimal('taco', exception: false)
+=> nil
+```
+
+Maybe that parsing logic ends up looking something like this:
+
+```ruby
+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.