Order Matters For rescue_from Blocks

In a Rails controller, you can declare any number of rescue_from blocks for capturing and responding to execeptions that are raised by your application.

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.