Get a flash message from a Rails Controller in a Sinatra action

get_flash.rb

def get_flash
  signed_message = request.cookies['_project_name_session']
  
  if signed_message.present?
    secret = ProjectName::Application.config.secret_token
    verifier = ActiveSupport::MessageVerifier.new(secret)
    session = verifier.verify(signed_message)

    @flash = session.delete('flash')

    signed_message = verifier.generate(session)
    response.set_cookie('_project_name_session', :value => signed_message, :path => '/')
  end
end

Make sure you're setting the path in response#set_cookie or you will have a very strange session bug.