ruby-on-rails - Is there a way to get session cookie name in rack middleware?

Question

I use middleware class in Rack, Ruby on Rails and Sinatra applications.

Rack application

config.ru:

use Rack::Session::Cookie, key: '_dummy_session', secret: '...'
use MyMiddleware
run Dummy.new

Ruby on Rails application

config/initializers/secret_token.rb:

Dummy::Application.config.secret_key_base = '...'

config/initializers/session_store.rb:

Dummy::Application.config.session_store :cookie_store, key: '_dummy_session'

config/initializers/my_middleware.rb:

Dummy::Application.config.middleware.use 'MyMiddleware'

Middleware class

lib/my_middleware.rb:

class MyMiddleware
  def initialize(app)
    @app = app
  end

  def call(env)
    ...
    @app.call(env)
  end
end

How can I get the name of session cookie ('_dummy_session') in MyMiddleware#call method?