Login from Omniauth.

Step-1  Put gem into gemfile.

             gem 'omniauth-facebook'
              gem 'omniauth-twitter'
       gem "omniauth-google-oauth2"

             gem  'omniauth' 

  and after that run bundle command on your console.

Step-2  App->Config->initializer-> omniauth.rb. 

                  copy and paste few line below given.

          

        Rails.application.config.middleware.use OmniAuth::Builder do

provider :facebook, '10370967297316xxx' , '6703b75235b704e789aa9fbb649xxxx'

         or set environment variable

  provider :google_oauth2, ENV["GOOGLE_CLIENT_ID"], ENV["GOOGLE_CLIENT_SECRET"]

provider :twitter, ENV['TWITTER_KEY'], ENV['TWITTER_SECRET']

end

after you will change some thing in your config file you have to restart the server again.

Step-3 you have to added migration 

        rails g migration AddOmniautoToUsers

  

        open migration file copy and paste this code

        

        add_column :users, :provider, :string

  add_column :users, :uid, :string

  add_column :users, :name, :string

  in console  type command -> rake db:migrate

step -4 user model 

    def self.sign_in_from_omniauth(auth)

       user = where(provider: auth['provider'], uid: auth['uid']).first_or_initialize 

       user.email =  auth['info']['email']

       user.name = auth['info']['name']

       user.password = Devise.friendly_token[0,20]

       user.confirmed_at = Time.now

       user.save

      user

  end

step -5 User controller 

           def omniauth

auth=request.env["omniauth.auth"]

                session[:omniauth] = auth.except('extra')

                user = User.sign_in_from_omniauth(auth)

                sign_in user

                redirect_to root_url, notice: "SIGNED IN"

end

Step-6  in routes.rb

             get 'auth/:provider/callback', to: "users#omniauth"

Step 7 - put this is link in view 

<%= link_to  "/auth/facebook" do %>Facebook<% end %>

<%= link_to 'Twitter', '/auth/twitter' %>

<%= link_to 'Google+', '/auth/google' %>