Add Order Matters For Rescue From Blocks as a rails til

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.