devise.rb

  config.omniauth :google_oauth2, Rails.application.secrets.google_oauth2_key, Rails.application.secrets.google_oauth2_secret,
    scope: 'email, profile', image_aspect_ratio: 'square', image_size: 300, access_type: 'online'

user.rb

  def self.find_for_oauth(auth, signed_in_resource = nil)
    identity = Identity.find_for_oauth(auth)
    user = signed_in_resource ? signed_in_resource : identity.user

    if user.nil?
      email = auth.info.email
      user = User.find_by(email: email) if email

      # Create the user if it's a new registration
      if user.nil?
        password = Devise.friendly_token[0,20]
        name = auth.info.name
        profile_picture = auth.info.image
        if auth.provider == 'facebook'
          user = User.new(
            email: email ? email : "#{auth.uid}@change-me.com",
            name: name,
            remote_avatar_url: profile_picture,
            password: password,
            password_confirmation: password
          )
        elsif auth.provider == 'twitter'
          user = User.new(
            email: email ? email : "#{auth.uid}@change-me.com",
            name: name,
            remote_avatar_url: profile_picture,
            password: password,
            password_confirmation: password
          )
        elsif auth.provider == 'google_oauth2'
          user = User.new(
            email: email ? email : "#{auth.uid}@change-me.com",
            name: name,
            remote_avatar_url: profile_picture,
            password: password,
            password_confirmation: password
          )
        end
      end
      #user.skip_confirmation!
      #user.save!   <-- this line is generating a conflict with usual (email-way) sign up
    end

    if identity.user != user
      identity.user = user
      identity.save!
    end

    return user
  end


  omniauth_callback_controller

  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(:success, :success, kind: provider_name("#{provider}")) if is_navigational_format?
        else
          session["devise.#{provider}_data"] = env["omniauth.auth"]
          redirect_to new_user_registration_url
        end
      end
    }
  end

  [:twitter, :facebook, :google_oauth2].each do |provider|
    provides_callback_for provider
  end

  def after_sign_in_path_for(resource) # Revisa después de cada login si el mail del usuario es válido
    if resource.email_verified?
      super resource # Acción por defecto de Devise (si no está configurada, va al root_path)
    else
      finish_signup_path(resource)
    end
  end

  def provider_name(provider)
    {twitter: 'Twitter', facebook: 'Facebook', google_oauth2: 'Google'}[provider.to_sym] || provider.capitalize
  end

end