Is here any possibility to modify devise SessionsController for ajax communication?

Edit

I found the solution, and posted it into answers, thanks

1. Generate Devise controllers so we can modify it

rails g devise:controllers

Now we have all controllers in the app/controllers/[model] directory

2. Edit routes.rb

Let's set Devise to use our modified SessionsController

First add this code (of course change :users to your devise model) into config/routes.rb

devise_for :users, controllers: {
  sessions: 'users/sessions'
}

3. Modify sessions_controller.rb

Find the create method and change it to

def create
  resource = User.find_for_database_authentication(email: params[:user][:email])
  return invalid_login_attempt unless resource

  if resource.valid_password?(params[:user][:password])
    sign_in :user, resource
    return render nothing: true
  end

  invalid_login_attempt
 end

Create new method after protected

def invalid_login_attempt
  set_flash_message(:alert, :invalid)
  render json: flash[:alert], status: 401
end

4. devise.rb

Insert this into config/initializers/devise.rb

config.http_authenticatable_on_xhr = false
config.navigational_formats = ["*/*", :html, :json]

5. Invalid email or password message

Insert a new message into config/locales/devise.en.yml under the sessions

invalid: "Invalid email or password."

6. View

= form_for resource, url: session_path(:user), remote: true do |f|
  = f.text_field :email
  = f.password_field :password
  = f.label :remember_me do
    Remember me
    = f.check_box :remember_me
  = f.submit value: 'Sign in'

:javascript
  $(document).ready(function() {
    //form id
    $('#new_user')
    .bind('ajax:success', function(evt, data, status, xhr) {
      //function called on status: 200 (for ex.)
      console.log('success');
    })
    .bind("ajax:error", function(evt, xhr, status, error) {
      //function called on status: 401 or 500 (for ex.)
      console.log(xhr.responseText);
    });
  });

Important thing remote: true

The reason why I am using status 200 or 401 unlike {status: 'true'} is less data size, so it is much faster and cleaner.

Explanation

On signing in, you get these data in params

action: "create"
commit: "Sign in"
controller: "users/sessions"
user: {
  email: "test@test.cz"
  password: "123"
  remember_me: "0"
}
utf8: "✓"

Before signing, you need to authorize the user.

resource = User.find_for_database_authentication(email: params[:user][:email])

User.find_for_database_authentication

If user is found, resource will be filled with something like

created_at: "2015-05-29T12:48:04.000Z"
email: "test@test.cz"
id: 1
updated_at: "2015-06-13T19:56:54.000Z"

Otherwise will be

null

If the user is authenticated, we are about to validate his password

if resource.valid_password?(params[:user][:password])

And finally sign in

sign_in :user, resource

Sources

SessionsController

Helped me Andreas Lyngstad