Rails 3: Ajax Link

cities_controller.rb

#FILE: app/controllers/cities_controller.rb

class CitiesController < ApplicationController

  #When you do a ajax request the respond handler is ':js'
  #So format.js will work. To render a response the new.js.erb 
  #file will be used.
  def new
    @city = City.new
    respond_to do |format|
      format.js   # It will look for new.js.erb
      format.html # It will look for new.html.erb
    end
  end


  #Even if you dont write the "respond_to" block
  #it will use proper render file based on request handler.
  #Calling edit via ajax => edit.js.erb
  #Calling edit via html => edit.html.erb 
  def edit
    @city = City.find(params[:id])
  end

end

index.html.erb

<!-- FILE: views/cities/index.html.erb -->
<!--
    In index page we will show "Create" and "Edit" link.
    Clicking on those links will show the form within the same page.

    Basically, we will have a content placeholder with id="main_content"
    where the form will be shown
-->

<div id="city_list">
    <!-- Show the index page content here -->
</div>

<%= link_to 'Create', new_city_path, :remote => true %> | 
<%= link_to 'Edit', edit_city_path(city), :remote => true %>

<div id="main_content">
</div>

new.js.erb

/*
  FILE: views/cities/new.js.erb
  FILE: views/cities/edit.js.erb
  Both has same content
*/
/*
   The form is rendered and stored in "comment_form" variable.
   Then the whole form is inserted into the DIV with id=main_content
*/

var comment_form = $('<%= j(render(:partial => "form"))%>');
$('#main_content').html(comment_form);