Add Order Matters For Rescue From Blocks as a rails til

Author: jbranchaud

README.md

@@ -9,7 +9,7 @@ and pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_904 TILs and counting..._
+_905 TILs and counting..._
 
 ---
 
@@ -560,6 +560,7 @@ _904 TILs and counting..._
 - [Mark For Destruction](rails/mark-for-destruction.md)
 - [Merge A Scope Into An ActiveRecord Query](rails/merge-a-scope-into-an-activerecord-query.md)
 - [Migrating Up Down Up](rails/migrating-up-down-up.md)
+- [Order Matters For `rescue_from` Blocks](rails/order-matters-for-rescue-from-blocks.md)
 - [Params Includes Submission Button Info](rails/params-includes-submission-button-info.md)
 - [Perform SQL Explain With ActiveRecord](rails/perform-sql-explain-with-activerecord.md)
 - [Polymorphic Path Helpers](rails/polymorphic-path-helpers.md)

rails/order-matters-for-rescue-from-blocks.md

@@ -0,0 +1,36 @@
+# Order Matters For `rescue_from` Blocks
+
+In a Rails controller, you can declare any number of [`rescue_from`
+blocks](https://api.rubyonrails.org/classes/ActiveSupport/Rescuable/ClassMethods.html)
+for capturing and responding to execeptions that are raised by your
+application.
+
+```ruby
+class BooksController < BaseController
+
+  rescue_from ForbiddenAction do |e|
+    render json: { error: e.message }.to_json, status: 403
+  end
+
+  rescue_from StandardError do |e|
+    render json: { error: e.message }.to_json, status: 500
+  end
+
+  def index
+    # ...
+
+    raise ForbiddenAction, "Which rescue_from is this going to hit?"
+  end
+end
+```
+
+The potential problem with above is the ordering of the two `rescue_from`
+blocks. Assume that `ForbiddenAction` is a subclass of the `StandardError`
+class -- this is likely the case for exceptions you declare in your app. The
+top `rescue_from` will never get hit because everything that subclasses
+`StandardError` will be trapped by the bottom `rescue_from`.
+
+These `rescue_from` blocks are applied bottom-up. That means you have to
+consider the class hierarchy when structuring your code. In the above code
+example, if we flip the two of them around, we will then get what we are
+expecting.