背景：

專案本身已使用devise來為使用者驗證，如今多加google oauth2驗證登入一項。

1. 加入omniauth-google-oauth2 Gem

$ subl Gemfile

gem 'omniauth-google-oauth2'

$ bundle install

2. API申請位置

Google：https://cloud.google.com/console

啟用”Contacts API” and “Google+ API”
注意：網址沒有 https://

$ rails g migration AddColumnsToUsers provider:string uid:string
因為omniauth 和 devise搭配，所以不需要為users再開欄位，目前夠用。

3. 將剛剛從google api申請的金鑰加進去

“config/initializers/devise.rb”

config.omniauth :google_oauth2, "APP_ID", "APP_SECRET", { }

4. 修改routes.rb，加入callback routes

“config/routes.rb”

devise_for :users, :controllers => { :omniauth_callbacks => "users/omniauth_callbacks" }

5. 為user model加入以下程式碼

“/app/models/user.rb”

devise :omniauthable, :omniauth_providers => [:google_oauth2]

以下為完整版程式碼：

  devise :database_authenticatable, :registerable,
    :recoverable, :rememberable, :trackable, :validatable,
    :omniauthable, :omniauth_providers => [:google_oauth2]

6. 創建callbacks controller 和新增相關程式碼

$ rails g controller OmniauthCallbacks

def google_oauth2       
    @user = User.from_omniauth(request.env["omniauth.auth"])
    sign_in_and_redirect @user
end

7. 去user model 做判斷，舊用戶直接登入，如果是新用戶就新增新帳號

$ subl app/model/user.rb

  def self.from_omniauth(auth)
    user = User.where(email: auth.info.email).first

    if user.name.blank?
      user.provider = auth.provider
      user.uid = auth.uid
      user.github_link = auth.info.image
      user.name = auth.info.name
      user.oauth_token = auth.credentials.token
      user.password = auth.credentials.token
      user.oauth_expires_at = Time.at(auth.credentials.expires_at)
      user.save!
      user
    else
      user.name = auth.info.name
      user.github_link = auth.info.image
      user
    end
  end

8. 製作一個「登入」按鈕

<%= link_to "Sign in with Google", user_omniauth_authorize_path(:google_oauth2) %>

參考資料：

https://github.com/zquestz/omniauth-google-oauth2
http://deepakrip007.wordpress.com/2013/11/05/google-integration-using-devise-and-omniauth-in-rails-app/

[Rails4] devise和google oauth實作登入

背景：