Get access to the Sinatra application instance that runs the current test and mock it

gistfile1.rb

require 'something/to/be/required'

class Sinatra::Base
  @@prepared = nil

  def self.onion_core
    onion = prototype
    loop do
      onion = onion.instance_variable_get('@app')
      return onion if onion.class == self || onion.nil?
    end
  end

  def self.prepare_instance
    @@prepared = onion_core
  end

  # Override
  def call(env)
    d = @@prepared || dup
    @@prepared = nil
    d.call!(env)
  end
end

describe 'An Sinatra app' do
  include Rack::Test::Methods
  
  def app
    Sinatra::Application
  end
  
  it 'mocks Kernel#open' do
    file = {}
    file.stub(:read).and_return('Happy New Year!')
    a = app.prepare_instance
    a.should_receive(:open).and_return(file)
    
    get '/new_year'
  end
end