Grouping lists of records

The SS_List class is designed to return a flat list of records. These lists can get quite long, and hard to present on a single list. Pagination is one way to solve this problem, by splitting up the list into multiple pages.

In this howto, we present an alternative to pagination: Grouping a list by various criteria, through the GroupedList class. This class is a SS_ListDecorator, which means it wraps around a list, adding new functionality.

It provides a groupBy() method, which takes a field name, and breaks up the managed list into a number of arrays, where each array contains only objects with the same value of that field. Similarly, the GroupedBy() method builds on this and returns the same data in a template-friendly format.

Grouping Sets By First Letter

This example deals with breaking up a SS_List into sub-headings by the first letter.

Let's say you have a set of Module objects, each representing a SilverStripe module, and you want to output a list of these in alphabetical order, with each letter as a heading; something like the following list:

*	B
		* Blog
		* CMS Workflow
		* Custom Translations
		* Database Plumber
		* ...

along with a method that returns the first letter of the title. This will be used both for grouping and for the title in the template.

	class Module extends DataObject {
		private static $db = array(
			'Title' => 'Text'
		);
	
		/**
		 * Returns the first letter of the module title, used for grouping.
		 * @return string
		 */
		public function getTitleFirstLetter() {
			return $this->Title[0];
		}
	}

sorted by title. For this example this will be a method on the Page class.

	class Page extends SiteTree {
	
		// ...
	
		/**
		 * Returns all modules, sorted by their title.
		 * @return GroupedList
		 */
		public function getGroupedModules() {
			return GroupedList::create(Module::get()->sort('Title'));
		}
	
	}

a number of sets, grouped by the field that is passed as the parameter. In this case, the getTitleFirstLetter() method defined earlier is used to break them up.

	<%-- Modules list grouped by TitleFirstLetter --%>
	<h2>Modules</h2>
	<% loop $GroupedModules.GroupedBy(TitleFirstLetter) %>
		<h3>$TitleFirstLetter</h3>
		<ul>
			<% loop $Children %>
				<li>$Title</li>
			<% end_loop %>
		</ul>
	<% end_loop %>

Grouping a set by month is a very similar process. The only difference would be to sort the records by month name, and then create a method on the DataObject that returns the month name, and pass that to the GroupedList::GroupedBy() call.

We're reusing our example Module object, but grouping by its built-in Created property instead, which is automatically set when the record is first written to the database. This will have a method which returns the month it was posted in:

	class Module extends DataObject {
	
		// ...
	
		/**
		 * Returns the month name this news item was posted in.
		 * @return string
		 */
		public function getMonthCreated() {
			return date('F', strtotime($this->Created));
		}
	
	}

sorted by month name from January to December. This can be accomplshed by sorting by the Created field:

	class Page extends SiteTree {
		
		// ...
		
		/**
		 * Returns all news items, sorted by the month they were posted
		 * @return GroupedList
		 */
		public function getGroupedModulesByDate() {
			return GroupedList::create(Module::get()->sort('Created'));
		}
	
	}
	// Modules list grouped by the Month Posted
	<h2>Modules</h2>
	<% loop $GroupedModulesByDate.GroupedBy(MonthCreated) %>
		<h3>$MonthCreated</h3>
		<ul>
			<% loop $Children %>
				<li>$Title ($Created.Nice)</li>
			<% end_loop %>
		</ul>
	<% end_loop %>