ruby on rails authentication using devise gem

scope 'admin' do
  resources :users
end

class Users::SessionsController < Devise::SessionsController
  def create
    resource = warden.authenticate!(scope: resource_name, recall: "#{controller_path}#new")
    set_flash_message(:notice, :signed_in) if is_navigational_format?
    sign_in(resource_name, resource)

    respond_to do |format|
      format.html do
        respond_with resource, location: redirect_location(resource_name, resource)
      end
      format.json do
        render json: { response: 'ok', auth_token: current_user.authentication_token }.to_json, status: :ok
      end
    end
  end
end 

authenticate :user, if: lambda { |u| u.email == "xyz@gmail.com" }

{ :if => lambda { |u| u.email ... } }

authenticate :user, lambda { |u| u.email == "xyz@gmail.com" } do
  mount Sidekiq::Web => '/sidekiq'
end