Introduction to Ruby on Rails with Ajax

The scaffolding we have for editing a slideshow shows just the slideshow attributes that are stored directly in the slideshows table: the slideshows name and the date on which it was created. The most important part is missing: the photos that are part of the slideshow!

By now, you've probably realized that this is because the scaffolding code deals with only one database table: the slideshows table. The relationship data about which photos are assigned to a slideshow and their order in the slideshow are stored in the slides table. Scaffolding does not handle relationships, so you have to write the code to edit this relationship data.

We're going to display a list of thumbnails of all the photos that are in a slideshow, and then let the user reorder them using drag-and-drop. If you've had to struggle through implementing drag-and-drop before, you're not going to believe how easy this is going to be. Here's a hint: this will take a total 34 additional lines of Ruby, CSS, and RHTML template!

Let's start by reviewing the current implementation of the edit action in the slideshow controller:

  def edit
    @slideshow = Slideshow.find(params[:id])
  end

This action expects to find the ID of the slideshow to edit passed in as the id parameter, which is normally decoded from the URL. You find the slideshow with that ID and assign that slideshow object to the instance variable @slideshow, so that it can be accessed in the view template.

That is really all that's needed here, so you won't have to add any code to this method. The changes will start with the edit view template, so edit the template photos/app/views/slideshows/edit.rhtml and make it look like this (the changes are in bold):

  <h1>Editing slideshow</h1>

  <%= link_to 'Play this Slideshow',
              :action => 'show', :id => @slideshow %>

  <div id='slideshow-contents'>
    <%= render :partial => 'show_slides_draggable' %>
  </div>
  <div id='slideshow-attributes'>
    <%= start_form_tag :action => 'update', :id => @slideshow %>
      <%= render :partial => 'form' %>
      <%= submit_tag 'Save Attributes' %>
    <%= end_form_tag %>
  </div>

Notice that the existing <%= render :partial => 'form' %> is wrapped in a <div> tag with an id attribute of slideshow-attributes. You will use this name in one of your CSS files to control how this section is displayed.

There is also a completely new section that displays thumbnails of the photos in the slideshow:

  <div id='slideshow-contents'>
     <%= render :partial => 'show_slides_draggable' %>
  </div>

This code also uses a <div> tag with an id attribute, for the same reason: to use a CSS file to control its appearance. This div also renders a new partial view template named show_slides_draggable, which we will create next.

Create the file photos/app/views/slideshows/_show_slides_draggable.rhtml with the following contents:

  <ol id='sortable_thumbs'>
     <% for slide in @slideshow.slides %>
        <li id='thumbs_<%= slide.id %>' class='slides'>
           <%= thumbnail_tag slide %>
        </li>
     <% end %>
  </ol>

  <%= sortable_element('sortable_thumbs',
                       :url => {:action => 'update_slide_order'}) %>

The first part is pretty standard stuff. We're creating an HTML ordered list, in which each list item is a thumbnail image of one of the photos in the slideshow (note that the thumbnail_tag helper function that was created earlier). However, it's the last two lines that do the heavy lifting.

sortable_element is a helper function that generates the JavaScript code that turns our list into a user-sortable, drag-and-drop-capable list. It wraps this list an HTML form, and the :url option specifies the URL to post to the server whenever the user changes the order of the list. In this case, it calls the action method update_slide_order in our slideshow controller. This call works in the background using an Ajax call.