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Listing 6-15. Index Action Updated in app/controllers/events_controller.rb
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def index @event_pages, @events = paginate :events, :per_page => 10 end
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Now delete the list action. This cuts down on the clutter. Also, using the index action as the collection action (wherein we display a list of resources) is a Rails convention not yet applied to the scaffold generator. We can apply another of the techniques you ve learned rendering a collection of partials to further improve the code.
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CHAPTER 6 ACTION PACK: WORKING WITH THE VIEW AND THE CONTROLLER
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PROBLEM: PAGINATION
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Pagination is the process of dividing up a result set into groups of pages, and you ll find examples of it all over the web. Google uses pagination to show you ten results per page for any query. Since it s so common in web development, Rails includes a pagination helper. Here s an example from the index action on the events controller.
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@event_pages, @events = paginate :events, :per_page => 10
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The paginate method returns a two-element array. In the preceding example, we re assigning each element to the @events_pages and @events instance variables, respectively. @events_pages contains the paginated object, and @events contains a collection of no more than the specified number of Event objects (in this case, 10). When it s time to display paginated results in a template, you can ask the current @events_pages object for its next and previous page and pass it to the controller via the :page parameter.
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link_to('Previous', :page => @events_pages.current.previous) link_to('Next', :page => @events_pages.current.next)
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The pagination helper works well for simple cases, but can suffer from efficiency problems when used for very large result sets. Kevin Clark has written about the perils of pagination in an article titled Things You Shouldn t Be Doing in Rails, which you can find at http://glu.ttono.us/ articles/2006/08/30/guide-things-you-shouldnt-be-doing-in-rails. For an alternative, more efficient approach, we like PJ Hyett and Chris Wanstrath s will_paginate plugin, which you can read about and download from http://errtheblog.com/ post/929. However, sometimes pagination isn t the best technique. One alternative proposed by Rails developer Pete Forde is endless scrolling. Read up on Pete s technique at http://unspace. ca/discover/pageless.
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Using Partials in the Events Templates
As you learned when walking through the users controller, Rails includes a clever way of economizing on template code called partials. The pattern of rendering a collection of partials is particularly powerful. Let s rename the list template of our events controller to index, and then rewrite it to take advantage of partials. Listing 6-16 shows the modified version.
CHAPTER 6 ACTION PACK: WORKING WITH THE VIEW AND THE CONTROLLER
Listing 6-16. Modified app/views/events/index.rhtml
<h1>All Events</h1> <%= render :partial => 'event', :collection => @events %> <%= link_to 'Previous page', { :page => @event_pages.current.previous } if @event_pages.current.previous %> <%= link_to 'Next page', { :page => @event_pages.current.next } if @event_pages.current.next %>
Listing 6-17 shows the _event partial.
Listing 6-17. The app/views/events/_event.rhtml File
<div class="event"> <h2><%= event.title %></h2> <ul> <li><%= event.occurs_on %></li> <li><%= event.location %></li> </ul> <div class="description"> <%= event.description %> </div> </div>
When this template is rendered, Rails will automatically iterate over the collection and create a div element for each event.
Adding Categories to the Events Form
In 5, we added categories to the Event model, but neither our controller nor our templates know about this yet. Let s remedy that now, starting with the events form. Add the code shown in bold in Listing 6-18 to the form partial in app/views/events/_form.rhtml.
Listing 6-18. Modified app/views/events/_form.rhtml
<%= error_messages_for 'event' %> <p><label for="event_title">Title</label><br/> <%= text_field 'event', 'title' %></p>
CHAPTER 6 ACTION PACK: WORKING WITH THE VIEW AND THE CONTROLLER
<p><label for="categories">Categories</label><br/> <%= select_tag( 'categories[]', options_for_select( Category.find(:all).collect {|c| [c.name, c.id] }, @event.category_ids ), :multiple => true ) %></p> <p><label for="event_location">Location</label><br/> <%= text_field 'event', 'location' %></p> <p><label for="event_occurs_on">Occurs on</label><br/> <%= date_select 'event', 'occurs_on' %></p> <p><label for="event_url">Url</label><br/> <%= text_field 'event', 'url' %></p> <p><label for="event_description">Description</label><br/> <%= text_area 'event', 'description', :rows => 5 %></p>
Note In order to fit within the page margins, we ve reformatted the code for the select_tag method.
It looks a little unwieldy, but bear with us.
The select_tag method deserves a closer look.
select_tag( 'categories[]', options_for_select( Category.find(:all).collect {|c| [c.name, c.id] }, @event.category_ids ), :multiple => true )
CHAPTER 6 ACTION PACK: WORKING WITH THE VIEW AND THE CONTROLLER
The select_tag method is another of Action View s form helpers. It creates a dropdown selection box, or if the :multiple option is set to true (as it is in our case), a multiple-choice selection box. By including a set of square brackets with the tag s name (categories[]), we re telling Rails to generate a collection of values in the params that are submitted. It s important to note that select_tag generates only the HTML select tag. We need to generate the actual option tags ourselves. Again, Rails provides a helper to take care of the heavy lifting: options_for_select. The options_for_select helper accepts a collection and returns a string of option tags. Given a collection where the elements respond to first and last (such as a two-element array), the firsts serves as the option test, and the lasts serves as the option values. A second argument can be passed to options_for_select to specify any options that should be preselected. In our case, we re passing in all the category_ids for the given event so they will be preselected. This is easily the most advanced helper we ve looked at so far, so it s okay to take it on faith a bit. Once we add the appropriate code to handle categories in the controller, everything should just work. You can find the full documentation for select_tag and options_for_select helpers in the Ruby API: http://api.rubyonrails.org/classes/ActionView/Helpers/
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