Rails 4 OmniAuth using Devise with Twitter, Facebook and Linkedin

app_config_initializers_devise.rb

Devise.setup do |config|
...
  config.omniauth :facebook, "KEY", "SECRET"
  config.omniauth :twitter, "KEY", "SECRET"
  config.omniauth :linked_in, "KEY", "SECRET"
...
end

app_controllers_application_controller.rb

class ApplicationController < ActionController::Base

  ...

  def ensure_signup_complete
    # Ensure we don't go into an infinite loop
    return if action_name == 'finish_signup'

    # Redirect to the 'finish_signup' page if the user
    # email hasn't been verified yet
    if current_user && !current_user.email_verified?
      redirect_to finish_signup_path(current_user)
    end
  end
end

app_controllers_omniauth_callbacks_controller.rb

class OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def self.provides_callback_for(provider)
    class_eval %Q{
      def #{provider}
        @user = User.find_for_oauth(env["omniauth.auth"], current_user)

        if @user.persisted?
          sign_in_and_redirect @user, event: :authentication
          set_flash_message(:notice, :success, kind: "#{provider}".capitalize) if is_navigational_format?
        else
          session["devise.#{provider}_data"] = env["omniauth.auth"]
          redirect_to new_user_registration_url
        end
      end
    }
  end

  [:twitter, :facebook, :linked_in].each do |provider|
    provides_callback_for provider
  end

  def after_sign_in_path_for(resource)
    if resource.email_verified?
      super resource
    else
      finish_signup_path(resource)
    end
  end
end

app_controllers_users_controller.rb

class UsersController < ApplicationController
  before_action :set_user, :finish_signup

  ...

  def finish_signup
    if request.patch? && params[:user] #&& params[:user][:email]
      if current_user.update(user_params)
        current_user.skip_reconfirmation!
        sign_in(current_user, :bypass => true)
        redirect_to current_user, notice: 'Your profile was successfully updated.'
      else
        @show_errors = true
      end
    end
  end
  
  private
    def set_user
      @user = User.find(params[:id])
    end

    def user_params
      accessible = [ :name, :email ] # extend with your own params
      accessible << [ :password, :password_confirmation ] unless params[:user][:password].blank?
      params.require(:user).permit(accessible)
    end
end

app_models_identity.rb

class Identity < ActiveRecord::Base
  belongs_to :user
  validates_presence_of :uid, :provider
  validates_uniqueness_of :uid, :scope => :provider

  def self.find_for_oauth(auth)
    identity = find_by(provider: auth.provider, uid: auth.uid)
    identity = create(uid: auth.uid, provider: auth.provider) if identity.nil?
    identity
  end
end

app_models_user.rb

class User < ActiveRecord::Base
  TEMP_EMAIL_PREFIX = 'change@me'
  TEMP_EMAIL_REGEX = /\Achange@me/

  # Include default devise modules. Others available are:
  # :lockable, :timeoutable
  devise :database_authenticatable, :registerable, :confirmable,
    :recoverable, :rememberable, :trackable, :validatable, :omniauthable

  validates_format_of :email, :without => TEMP_EMAIL_REGEX, on: :update

  def self.find_for_oauth(auth, signed_in_resource = nil)

    # Get the identity and user if they exist
    identity = Identity.find_for_oauth(auth)

    # If a signed_in_resource is provided it always overrides the existing user
    # to prevent the identity being locked with accidentally created accounts.
    # Note that this may leave zombie accounts (with no associated identity) which
    # can be cleaned up at a later date.
    user = signed_in_resource ? signed_in_resource : identity.user

    # Create the user if needed
    if user.nil?

      # Get the existing user by email if the provider gives us a verified email.
      # If no verified email was provided we assign a temporary email and ask the
      # user to verify it on the next step via UsersController.finish_signup
      email_is_verified = auth.info.email && (auth.info.verified || auth.info.verified_email)
      email = auth.info.email if email_is_verified
      user = User.where(:email => email).first if email

      # Create the user if it's a new registration
      if user.nil?
        user = User.new(
          name: auth.extra.raw_info.name,
          #username: auth.info.nickname || auth.uid,
          email: email ? email : "#{TEMP_EMAIL_PREFIX}-#{auth.uid}-#{auth.provider}.com",
          password: Devise.friendly_token[0,20]
        )
        user.skip_confirmation!
        user.save!
      end
    end

    # Associate the identity with the user if needed
    if identity.user != user
      identity.user = user
      identity.save!
    end
    user
  end

  def email_verified?
    self.email && self.email !~ TEMP_EMAIL_REGEX
  end
end

app_views_users_finish_signup.html.erb

<div id="add-email" class="container">
  <h1>Add Email</h1>
  <%= form_for(current_user, :as => 'user', :url => finish_signup_path(current_user), :html => { role: 'form'}) do |f| %>
    <% if @show_errors && current_user.errors.any? %>
      <div id="error_explanation">
        <% current_user.errors.full_messages.each do |msg| %>
          <%= msg %><br>
        <% end %>
      </div>
    <% end %>
    <div class="form-group">
      <%= f.label :email %>
      <div class="controls">
        <%= f.text_field :email, :autofocus => true, :value => '', class: 'form-control input-lg', placeholder: 'Example: email@me.com' %>
        <p class="help-block">Please confirm your email address. No spam.</p>
      </div>
    </div>
    <div class="actions">
      <%= f.submit 'Continue', :class => 'btn btn-primary' %>
    </div>
  <% end %>
</div>

config_environments_[environment].rb

...
  # Email
  config.action_mailer.delivery_method = :smtp
  config.action_mailer.perform_deliveries = true
  config.action_mailer.default_url_options = { :host => config.app_domain }
  config.action_mailer.smtp_settings = {
    address: 'smtp.gmail.com', 
    port: '587',
    enable_starttls_auto: true,
    user_name: 'someuser',
    password: 'somepass',
    authentication => :plain,
    domain => 'somedomain.com'
  }
...

config_routes.rb

...
  devise_for :users, :controllers => { omniauth_callbacks: 'omniauth_callbacks' }
...

config_routes_2.rb

...
  match '/profile/:id/finish_signup' => 'users#finish_signup', via: [:get, :patch], :as => :finish_signup
...

Gemfile

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

Generate migrations and models

rails generate devise:install
rails generate devise user
rails g migration add_name_to_users name:string
rails g model identity user:references provider:string uid:string

# Modify the db/migrate/[timestamp]_add_devise_to_users.rb to configure the Devise modules you will use.
# We usually enable the "confirmable" module when enabling email signups.

http://sourcey.com/rails-4-omniauth-using-devise-with-twitter-facebook-and-linkedin/