gistfile1.rb

  def authorize!(oauth_hash, user)
    User::Oauth2Service.new(oauth_hash, user).workflow self do
      # Account successfully created, or user successfully logged in. Now sign in user.
      on_sign_in do |account, provider_title|
        session.delete("omniauth.auth")
        session.delete("provider_title")

        if account.user.confirmed?
          sign_in_user_and_redirect(account.user, provider_title)
        else
          redirect_to users_registration_done_path and return
        end
      end

      # In some cases oauth2 provider does not provide required data (e.g. email)
      # So here we request for required fields
      on_invalid_credentials do |user, provider_title|
        @user = user
        @provider_title = provider_title

        # Keep oauth hash in session
        session["omniauth.auth"]  = request.env["omniauth.auth"]
        session["provider_title"] = @provider_title

        render :new
      end
    end
  end