Omniauth

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Installing

These notes are assuming you're also allowing regular email/password logins. It's greatly simplified if you don't...
::

rails generate model Authorization provider:string uid:string user_id:integer

Gemfile

gem 'omniauth'
gem 'omniauth-twitter'
gem 'omniauth-facebook'
gem 'omniauth-linkedin'

config/initializers/omniauth.rb

Rails.application.config.middleware.use OmniAuth::Builder do
  provider :twitter, 'CONSUMER_KEY', 'CONSUMER_SECRET'
  provider :facebook, 'APP_ID', 'APP_SECRET'
  provider :linked_in, 'CONSUMER_KEY', 'CONSUMER_SECRET'
end

app/models/user.rb

add:

has_many :authorizations

def add_provider(auth_hash)
  unless authorizations.find_by_provider_and_uid(auth_hash["provider"], auth_hash["uid"])
        Authorization.create :user => self, :provider => auth_hash["provider"], :uid => auth_hash["uid"]
  end
end

app/controllers/sessions_controller.rb

def omniauth_create
  auth_hash = request.env['omniauth.auth']
  if session[:user_id]
        # Means user is signed in. Add the authorization to the user
        user = User.find(session[:user_id])
        user.add_provider(auth_hash)
  else
        auth = Authorization.find_or_create(auth_hash)
        # Create the session
        session[:user_id] = auth.user_id
        user = User.find(session[:user_id])
  end
  sign_in user
  redirect_back_or user
end

app/models/authorization.rb

belongs_to :user
validates :provider, :uid, :presence => true

def self.find_or_create(auth_hash)
  unless auth = find_by_provider_and_uid(auth_hash["provider"], auth_hash["uid"])
        user = User.find_by_email(auth_hash["info"]["email"])
        if not user
          # If it's a new user, we want to give them a solid password
          random_string = SecureRandom.base64(30)
          user = User.create :name => auth_hash["info"]["name"],
       					   :email => auth_hash["info"]["email"],
       					   :password => random_string,
       					   :password_confirmation => random_string
        end
        auth = create :user_id => user, :provider => auth_hash["provider"], :uid => auth_hash["uid"]
  end
  auth
end

config/routes.rb

match '/auth/:provider/callback', to: 'sessions#create'
match '/auth/failure', to: 'sessions#failure'