Google Integration Using Devise and Omniauth In Rails App.

Step:1
Add the gems in your gem file

gem ‘devise’
gem 'omniauth'
gem 'omniauth-google-oauth2'

Run the “bundle install” command to install the gem.

Step:2
You need two more columns to store provider type and userid given from google

rails g migration AddProviderToUsers provider:string uid:string

Runt rake db:migrate to insert the columns in users table.

Step:3
Go the user model “user.rb” and add the following line

devise : omniauthable

Step:4
First of all you need to create an app in google to get “Client key” and “Client Secret key”

https://code.google.com/apis/console/

Create an app and get the Client id and secret key.

Step:5
Now you need to declare the provider name and client id and key.Go to the file config/initializers/devise.rb and the following line

require 'omniauth-google-oauth2'
config.omniauth :google_oauth2, "APP_ID", "APP_SECRET", { access_type: "offline", approval_prompt: "" }

Step:6
Go to your layout file and the following block


<% if user_signed_in? %>
Signed in as <%= current_user.name %>. Not you?
<%= link_to "Sign out", destroy_user_session_path,:method => :delete %>
<% else %>
<%= link_to "Sign up", new_user_registration_path %> or
<%= link_to "Sign in", new_user_session_path %>
<%= link_to "Sign in with Google", user_omniauth_authorize_path(:google_oauth2) %>
<% end %>

Step:7
Before creating the call back method change your route like below


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

Step:8
Create a new controller named as “omniauth_callbacks_controller.rb”.Add the following method in it.


class OmniauthCallbacksController < Devise::OmniauthCallbacksController  
def google_oauth2
    
    @user = User.find_for_google_oauth2(request.env["omniauth.auth"], current_user)

    if @user.persisted?
      flash[:notice] = I18n.t "devise.omniauth_callbacks.success", :kind => "Google"
      sign_in_and_redirect @user, :event => :authentication
    else
      session["devise.google_data"] = request.env["omniauth.auth"]
      redirect_to new_user_registration_url
    end
  end

Step:9
Add the following block in your user model.


def self.find_for_google_oauth2(access_token, signed_in_resource=nil)
    data = access_token.info
    user = User.where(:provider => access_token.provider, :uid => access_token.uid ).first
    if user
      return user
    else
      registered_user = User.where(:email => access_token.info.email).first
      if registered_user
        return registered_user
      else
        user = User.create(name: data["name"],
          provider:access_token.provider,
          email: data["email"],
          uid: access_token.uid ,
          password: Devise.friendly_token[0,20],
        )
      end
   end
end