gem 'devise'
gem 'omniauth-facebook'

bundle install

rails g devise:install
      create  config/initializers/devise.rb
      create  config/locales/devise.en.yml

rails g devise:controllers users
      create  app/controllers/users/confirmations_controller.rb
      ~~~
      create  app/controllers/users/omniauth_callbacks_controller.rb

rails g devise:views
      invoke  Devise::Generators::SharedViewsGenerator
      create    app/views/devise/shared
      ~~~
      create    app/views/devise/mailer/unlock_instructions.html.erb

rails g devise User
      invoke  active_record
      create    db/migrate/20170708090421_devise_create_users.rb
      ~~~
       route  devise_for :users

rails db:migrate

class User < ApplicationRecord
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable
end

config.omniauth :facebook, ENV['APP_ID'], ENV['APP_SECRET']

class User < ApplicationRecord
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable, :omniauthable
end

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

rails g migration add_columns_to_users provider uid name

rails db:migrate

class ApplicationController < ActionController::Base
  before_action :configure_permitted_parameters, if: :devise_controller?

  protected

  def configure_permitted_parameters
    devise_parameter_sanitizer.permit(:sign_up, keys: [:name])
    devise_parameter_sanitizer.permit(:account_update, keys: [:name])
  end
end

class User < ApplicationRecord
  validates :name, presence: true, length: { maximum: 15 }, uniqueness: { case_sensitive: false }, format: { with: /\A[a-z0-9]+\z/i, message: "is must NOT contain any other characters than alphanumerics." }
end

class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def facebook
    # You need to implement the method below in your model (e.g. app/models/user.rb)
    #@user = User.from_omniauth(request.env["omniauth.auth"])

    # ユーザ登録まで自動的に実施せず、ユーザ検索のみ実施するように変更
    # メソッドはuser.rb側で定義します。
    @user = User.find_omniauth(request.env["omniauth.auth"])

    #if @user.persisted?
    # 新規ユーザの場合、この時点ではDBレコードが存在しないので以下に変更
    if @user
      sign_in_and_redirect @user #this will throw if @user is not activated
      set_flash_message(:notice, :success, :kind => "Facebook") if is_navigational_format?
    else
      # Facebookから取得した情報をsessionに格納
      session["devise.facebook_data"] = request.env["omniauth.auth"]
      #redirect_to new_user_registration_url

      # 新規ユーザの場合、`ユーザ名`登録用のテンプレートをrender
      @user = User.new()
      render 'devise/registrations/after_omniauth_signup'
    end
  end

  def failure
    redirect_to root_path
  end
end

class User < ApplicationRecord
  # 公式のサンプル(今回は使用しません)
  def self.from_omniauth(auth)
    where(provider: auth.provider, uid: auth.uid).first_or_create do |user|
      user.email = auth.info.email
      user.password = Devise.friendly_token[0,20]
      user.name = auth.info.name   # assuming the user model has a name
      user.image = auth.info.image # assuming the user model has an image
      # If you are using confirmable and the provider(s) you use validate emails,
      # uncomment the line below to skip the confirmation emails.
      # user.skip_confirmation!
    end
  end

  # 今回使用するメソッド
  def self.find_omniauth(auth)
    User.where(provider: auth.provider, uid: auth.uid).first
  end
end

class User < ApplicationRecord
  def self.new_with_session(params, session)
    super.tap do |user|
      if data = session["devise.facebook_data"] && session["devise.facebook_data"]["extra"]["raw_info"]
        user.email = data["email"] if user.email.blank?
      end
      if data = session["devise.facebook_data"]
        user.provider = data["provider"] if user.provider.blank?
        user.uid = data["uid"] if user.uid.blank?
        user.password = Devise.friendly_token[0,20] if user.password.blank?
      end
    end
  end
end

<%= form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f| %>

<%= link_to 'Log in', new_user_session_path %>
<%= link_to 'Sign up', new_user_registration_path %>

<%= link_to 'Sign up with Facebook', user_facebook_omniauth_authorize_path %>

<%= link_to 'Log out', destroy_user_session_path, method: :delete %>