Observing errors from middleware

application.rb

module MyApp
  class Application < Rails::Application
    if Rails.env == 'test'
      require 'diagnostic'
      config.middleware.use(MyApp::DiagnosticMiddleware)
    end

#     If previous code is not working then write code this way
#     if Rails.env == 'test'
#       require File.expand_path('../diagnostic', __FILE__)
#       config.middleware.use(MyApp::DiagnosticMiddleware)
#     end
  end
end

diagnostic.rb

module MyApp
  class DiagnosticMiddleware
    FILENAME = 'diagnostic.txt'

    def initialize(app)
      @app = app
    end

    def call(env)
      return @app.call(env)
    rescue StandardError => e
      trace = e.backtrace.select{ |l|l.start_with?(Rails.root.to_s) }.join("\n    ")
      msg = "\n#{e.class}\n#{e.message}\n#{trace}"
      File.open(FILENAME, 'a') { |f| f.write msg }
      raise e
    end
  end
end

# Another way to implement :
# To print in stdout:

# module MyApp
#   class DiagnosticMiddleware

#     def initialize(app)
#       @app = app
#     end

#     def call(env)
#       return @app.call(env)
#     rescue StandardError => error
#       puts error.message
#       puts error.backtrace
#       raise error
#     end
#   end
# end


# Another way to implement DiagnosticMiddleware:
# module MyApp
#   class DiagnosticMiddleware
#     FILENAME = File.join(Rails.root, 'log', 'test.log')

#     def initialize(app)
#       @app = app
#     end

#     def call(env)
#       return @app.call(env)
#     rescue StandardError => e
#       trace = e.backtrace.select{ |l|l.start_with?(Rails.root.to_s) }.join("\n    ")
#       msg = "#{e.class}\n#{e.message}\n#{trace}\n"
#       File.open(FILENAME, 'a') { |f| 
#         f.write "\n"
#         f.write "\n"
#         f.write " ---   BACKTRACE   --- \n"
#         f.write "\n"

#         f.write msg 

#         f.write "\n"
#         f.write " ---   BACKTRACE   --- \n"
#         f.write "\n"
#         f.write "\n"
#       }
#       raise e
#     end
#   end
# end

I got these codes from Steve Masterman public gist. The gist link: https://gist.github.com/steve9001/1443408/3917792198d7f93b129c6009fcc9292ec4b7b664