OAuth2 with Devise

add_facebook_uid_to_users.rb

class AddFacebookUidToUsers < ActiveRecord::Migration
  def change
    add_column :users, :facebook_uid, :string
  end
end

initializer.rb

  # OAuth
  config.omniauth :facebook, "306897372733415", "178333649c7a3ae4842038a110e36e19",
    {:scope => 'email, offline_access', :client_options => {:ssl => {:ca_file => '/usr/lib/ssl/certs/ca-certificates.crt'}}} 

omniauth_callbacks_controller.rb

class OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def facebook
    @user = User.find_or_create_from_auth_hash(request.env["omniauth.auth"])

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

routes.rb

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

user.rb

  def self.find_or_create_from_auth_hash(auth)
    user = self.where(facebook_uid: auth.uid).first

    
    unless user
      user = User.find_or_create_by_email(
        facebook_uid: auth.uid,
        email: auth.info.email,
        password: Devise.friendly_token[0,20],
        name: auth.info.name,
        bio: auth.info.description,
        city: auth.info.location,
        facebook_link: "https://www.facebook.com/#{auth.info.nickname}"
      )
      user.remote_avatar_url = "https://graph.facebook.com/#{auth.info.nickname}/picture?type=large"
      user.save
    end

    user
  end

view.html.erb

      <%- if devise_mapping.omniauthable? %>
        <ul>
        <%- resource_class.omniauth_providers.each do |provider| %>
          <li><%= link_to "#{t('app.devise.shared.links.sign.with')} #{provider.to_s.titleize}", omniauth_authorize_path(resource_name, provider), class: "btn-login #{provider.to_s.downcase}" %></li>
        <% end -%>
        </ul>
      <% end -%>