参考

Twiwt:Blog / jugyo : OmniAuth で簡単 Twitter 認証！

Omniauth NoMethodError in SessionsController - Stack Overflow

Gemfileに以下を追記後、bundle install

gem 'omniauth'
gem 'omniauth-twitter'

その後

bundle install

Twitterのconsumer key/sercret を記入

config/initializers/omniauth.rb に取得したKeyを入れる。

omniauth.rb

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

SessionsControllerを作成

rails g controller sessions

app/controllers/sessions_controller.rbを以下に編集

sessions_controller.rb

class SessionsController < ApplicationController
  def callback
    auth = request.env["omniauth.auth"]
    user = User.find_by_provider_and_uid(auth["provider"], auth["uid"]) || User.create_with_omniauth(auth)
    session[:user_id] = user.id
    redirect_to root_url, :notice => "Signed in!"
  end

  def destroy
    session[:user_id] = nil
    redirect_to root_url, :notice => "Signed out!"
  end
end

config/routes.rb に認証のルーティングを記入

routes.rb

match '/auth/:provider/callback' => 'sessions#callback'
match "/signout" => "sessions#destroy", :as => :signout

User モデルの作成

rails g model user

db/migrate/XXXXXXXXXXXXXX_create_users.rb を編集。

XXXXXXXXXXXXXX_create_users.rb

class CreateUsers < ActiveRecord::Migration
  def self.up
    create_table :users do |t|
      t.string :provider, :null => false
      t.string :uid, :null => false
      t.string :screen_name, :null => false, :uniq => true
      t.string :name, :null => false

      t.timestamps
    end
    add_index :users, [:provider, :uid]
    add_index :users, [:screen_name]
  end

  def self.down
    drop_table :users
  end
end

app/models/user.rbを編集。

user.rb

class User < ActiveRecord::Base
  def self.create_with_omniauth(auth)
    create! do |user|
      user.provider = auth["provider"]
      user.uid = auth["uid"]
      user.name = auth["info"]["name"]
      user.screen_name = auth["info"]["nickname"]
    end
  end
end

編集後、migrate

rake db:migrate

ApplicationControllerを編集

app/controllers/application_controller.rb を編集

application_controller.rb

class ApplicationController < ActionController::Base
  ...

  helper_method :current_user

  private

  def current_user
    @current_user ||= User.find(session[:user_id]) if session[:user_id]
  end
end

Viewの作成

rails g controller login index

app/views/login/index.html.erb に以下を追加

index.html.erb

<% if current_user %>
  Welcome <%= current_user.name %>!
  <%= link_to "Sign Out", signout_path %>
<% else %>
  <%= link_to "Sign in with Twitter", "/auth/twitter" %>
<% end %>

config/routes.rbを編集

routes.rb

root :to => "login#index"

public/index.htmlがあれば、削除。

rails s

でサーバー起動。
そののち、http://localhost:3000/ にアクセスでログインできるか確認できる。