Paginating a filtered DataObjectSet

Tweet

25 October 2010 | Marcus Dalgren | DataObjects, Templates | Supports v2.4

13 9

 Source files (336 bytes)

If you have ever tried to paginate a DataObjectSet that has been customised since fetching it from the database you will have descovered that it simply doesn't work. This short tutorial provides some great code to resolve this problem and paginate to your hearts content!

The problem

In its standard implementation DataObjectSet does not allow pagination of a set that has been filtered after the set has been fetched from the database.So if you have a function called getNewsArticles() in your model that looks like this´

function getNewsArticles() {
	$newsArticles = DataObject::get("NewsArticle");
	$doSet = new DataObjectSet();
	foreach ($newsArticles as $newsArticle) {
		if ($newsArticle->canView()) {
			$doSet->push($newsArticle);
		}
	}
	return $doSet;
}

Then you can't paginate it using DataObjectSets built in pagination controls. The built in controls make the assumption that the set that's being paginated is the set that has been fetched from the database and that the SQL LIMIT clause is the basis for the page size. When you filter a set after fetching it from the database then using the LIMIT clause won't work because we don't know how many records we'll end up with after filtering.

The solution

In order to make filtered pagination work we will decorate DataObjectSet with our own pagination method. When you decorate a class all instantiated objects of that class will have this method available for use. The decorated method will slice out the records we need for the current page. The decorator looks like this

class DataObjectSetExtension extends Extension {
	public function Pagination() {
		$pageLimits = $this->owner->getPageLimits();
		$items = $this->owner->toArray();
		$items = array_slice($items, $pageLimits["pageStart"], $pageLimits["pageLength"]);
		return new DataObjectSet($items);		
	}	
}

What the code does is fetch the records as an array from the owner class (in this case DataObjectSet) and then slice out the records it needs based on the pageLimits set on the DataObjectSet.

In order to make the method available on all our DataObjectSets the following line needs to be put in _config.php

Object::add_extension('DataObjectSet', 'DataObjectSetExtension');

After flushing your site the method will now be available on all DataObjectSets. In order to get the getNewsArticles() function to work with our pagination it nees to be modified. The modified function looks like this

function getNewsArticles() {
	if(!isset($_GET['start']) || !is_numeric($_GET['start']) || (int)$_GET['start'] < 1) $_GET['start'] = 0;
	$SQL_start = (int)$_GET['start'];
	$newsArticles = DataObject::get("NewsArticle");
	$doSet = new DataObjectSet();
	foreach ($newsArticles as $newsArticle) {
		if ($newsArticle->canView()) {
			$doSet->push($newsArticle);
		}
	}
	$doSet->setPageLimits($SQL_start, 30, $doSet->Count());
	return $doSet;
}

As you can see, some things have changed. First of all, we're checking for a GET variable called start. This is the standard pagination variable for DataObjectSet but you can change it if you want with DataObjectSet->setPaginationGetVar($var).

The fetching and filtering is done exactly as before but before returning the set we need to explicitly set the page limits with the setPageLimits() function. If setPageLimits() is not set then the Pagination method won't know what values to use. In this case I have set the start value to the GET variable, page size to 30 and the total size to the number of records in the set after the filtering has been done.

When displaying the news articles in the template we loop over the Pagination method of the DataObjectSet instead of just looping over the getNewsArticles() function like we normally do. Note that you only use the Pagination method for printing the actual results, not for creating the pagination controls. The code below is mostly taken from the SilverStripe pagination tutorial that can be found here.

<% control NewsArticles.Pagination %>
<!--
	Put whatever you want to display here
-->
<% end_control %>
<% if NewsArticles.MoreThanOnePage %>
	<p>

	<% if NewsArticles.PrevLink %>
		<a href="$NewsArticles.PrevLink">&lt;&;lt; Prev</a> | 
	<% end_if %>

	<% control NewsArticles.Pages %>
	<% if CurrentBool %>
		<strong>$PageNum</strong> 
	<% else %>
		<a href="$Link" title="Go to page $PageNum">$PageNum</a> 
	<% end_if %>
	<% end_control %>

	<% if NewsArticles.NextLink %>
		| <a href="$NewsArticles.NextLink">Next &gt;&gt;</a>
	<% end_if %>

	</p>
<% end_if %>

As you can see in the snippet above we still use the regular set for the pagination controls but we use the Pagination method for actually displaying the records.

So to recap

1. Put the DataObjectSetExtension.php in your code folder
2. Add the line Object::add_extension('DataObjectSet', 'DataObjectSetExtension') to _config.php and flush your site
3. Fetch the data, filter it and set the page limits
4. Use the Pagination method to show your stuff

That's all, happy coding!