Devise ajax

devise_sessions_controller.rb

class DeviseSessionsController < Devise::SessionsController
  clear_respond_to
  respond_to :json
end

forms.html.erb

<h2>Sign in</h2>
<%=simple_form_for :user, url: session_path(:user), remote: true, html: {id: 'user_sign_in'} do |f| %>
  <%= f.input :username %>
  <%= f.input :password %>
  <%= f.button :submit %>
<%end%>

<h2>Sign up</h2>
<%=simple_form_for :user, url: registration_path(:user) do |f|%>
  <%= f.input :username %>
  <%= f.input :password %>
  <%= f.input :password_confirmation %>
  <%= f.button :submit %>
<%end%>

registrations_controller.rb

class RegistrationsController < Devise::RegistrationsController
  clear_respond_to
  respond_to :json
end

routes.rb

Rails.application.routes.draw do
  #sobrescribir los controllers de devise que partician en el login / sign in para que respondan json
  devise_for :users, :controllers => { omniauth_callbacks: "users/omniauth_callbacks", sessions: 'devise_sessions', registrations: 'registrations' }
  devise_scope :user do
    post '/users/fbasync' => "users/omniauth_callbacks#facebook_async"
  end
end