before_filter :cookie_required, :only => [:some_actions]

before_filter :login_required, :only => [:some_actions]

class Identity 
Whenever your controllers need to link to the user, do it through the identity instead.

@identity.user
What if the user's browser did not enable cookies?
Good question. If that happens, we will treat the protected region as private automatically. There's no need for you to handle it. The code is created to degrade gracefully.
Implementation of persistent cookie authentication
The implementation is as suggested by Barry Jaspan and Charles Miller. In fact, Drupal's Persistent Login Module follows this set of implementation.
Special cases
There's a special case you need to take note. For example, an anonymous user comes to your site, and place a few items on the cart. And then she registers as a new user. Will the data in her shopping cart be lost?  The gem will ensure that her cookie entry and her newly registered user entry will be transferred to the same identity. But as I do not know how your website will be implemented, I did not implement the merging of the data from her cart. You'll need to add your code to transfer the data to her new identity. An example of the implementation is here. You'll need to modify the merge_identity function from user_controller.rb  From app/controllers/user_controller.rb

def mergeIdentity(anonymousCookieValue, knownIdentityID)
if anonymousCookieValue == nil || knownIdentityID == nil
return false
end

return false if !LoginCookie.isAnonymous(anonymousCookieValue)
loginValue = LoginCookie.getCookieLogin(anonymousCookieValue)

#getting the both anonymous user's identity and the known identity
anonymousCookie = LoginCookie.find(:first, :conditions => ["login = ?", loginValue])
anonymousIdentity = anonymousCookie.identity
knownIdentity = Identity.find(knownIdentityID)
return false if anonymousIdentity == nil || knownIdentity == nil

#transferring the cookies
anonymousIdentity.login_cookies.each { |cookie|
cookie.identity_id = knownIdentity.id
cookie.save
}

#transferring the cards
anonymousIdentity.cards.each { |card|
card.identity_id = knownIdentity.id
card.save
}

#finally send the identity to its grave
anonymousIdentity.destroy

return true

end
This is about all you will need to know to implement the persistent cookie authentication system effectively. Any bugs or unclear points, feel free to contact me.

gem install persistent_cookie_authentication_generator

ruby script/generate persistent_cookie_authentication

rake db:migrate

include UserSystem

require 'environments/user_environment'

require 'smtp_tls'

ActionMailer::Base.smtp_settings = {:address => "smtp.gmail.com",
:port => "587",
:domain => "localhost.localdomain",
:authentication => :plain,
:user_name => "yourgmailusername",
:password => "yourgmailpassword"
}

Inflector.inflections do |inflect|#   inflect.plural /^(ox)$/i, '1en'
#   inflect.singular /^(ox)en/i, '1'
#   inflect.irregular 'person', 'people'
#   inflect.uncountable %w( fish sheep )
inflect.irregular 'LoginCookie', 'LoginCookies'
end

rake db:test:clone

rake test

ActionController::Base.session_options[:session_expires] = 10.minutes.from_now