Faceted search in WordPress

About a month ago I talked to a friend of mine about WordPress and PHP. Being a staunch .NET loyalist he quickly concluded that he had no time for sloppy PHP.

PHP supports both procedural and object-oriented paradigms and WordPress is a prime example of why this is not always a good idea: The WordPress API is partly based on procedures and partly based on global accessible God objects. Pragmatic – but not pretty.

But this do not mean that you cannot create a tidy and strictly object-oriented WordPress plugin. In this post I’ll show you how to structure your plugins.

The plugin that I am going to showcase is a plugin that add facets to WordPress’ search. We made this plugin for at The Royal Library for at Danish music research portal and its a pretty neat piece of software – clean and tightly coded and reasonably fast.

Faceted search explained

You probably know search facets from Google. Google’s result page allow you to show only certain types (or facets) of results – like images or videos.

You choose which facet to show by clicking on one of the buttons in the right side panel. Google will then show only the selected result type – filtering out all others.

Search facets makes Google simple to use: You just enter a search string and click search.

You do not have to choose what kind of results you are looking for before you get the actual results.

Understanding the WordPress search

The anatomy of a WordPress search is simple. When you enter the search string foobar and click search, WordPress requests the URL /?s=foobar.

Every time WordPress receives a request where the key s is set to some value the request is treated as a search and redirected to the result page.

The wp_query object

The results is fetched from the data layer using the object wp_query. The wp_query maintains a collection of predefined search conditions in key/value collection called query variables. The collection is manipulated by the two methods set and get.

The wp_query works by example: Most predefined query variables mirror the properties of a WordPress page. If you e.g. wish to fetch all pages with the title Frontpage you set the query variable post_title to Frontpage.

But the wp_query also includes some special query variables not directly related to page properties. The search uses its own variable s to do full text searches. After this variable has been set the search continues and calls the method get_post.

The method get_posts execute the following steps:

1. Cleans and sanitizes the query variable by calling the method parse_query.
2. Fires the WordPress event pre_get_posts.
3. Translate the query variables into a SQL SELECT query.
4. Execute the SQL query and fetch the result. The database interaction is done by the data abstraction object wpdb.

Implementing faceted search in WordPress

My plugin consist of two parts: A widget and a applier. The widget shows a list of all facets on the search result page:

Each facet is identified by an id, and when the user click a facets this id is added to the query string of a new search /s=foobar&facet_id=4.

Hence my facets are not filters but instead extra conditions that are added to the wp_query before performing a second search: It is most cases it is simply faster to let the database do another search than to start filtering the existing results in PHP.

The signature of a facet

I have implemented three types of facet – but their all inherit from the same base class:

What differentiates the three types are the add_facet method. It takes the wp_query and the key/value pair to it – but in slightly different ways.

Pages where a property has a specific value

The most basic type of facet simply takes the key/value pair and add it to the list of query variables:

class Property_Facet extends Facet
    /* (non-PHPdoc)
    * @see Facet::add_facet()
    public function add_facet(&$wp_query)
        // Adding value to wp_query
        $wp_query->set($this->key, $this->value);

This type can be used to create a many different facets:

$a = new Property_Facet();
$a->key = 'post_type';
$a->value = 'post';

$b = new Property ();
$b->key = 'post_parent';
$b->value = '230';

$c = new Property ();
$c->key = 'post_author';
$c->value = '25';

Facet $a shows only blog posts, facet $b shows only child pages of a specific page and facet $c shows only content from a specific author.

Understanding post_type property

WordPress’s two types of content – pages and posts – are both stored in the wp_posts table. The post_type property is used to differentiate the two.

When WordPress introduced custom types (e.g. post_type='article' or post_type='record') a set of method was needed that would work on all post types. The result is a rather confusing API where the term post can mean content with post_type='post' as well as all kind of content in the wp_posts table.

Pages that have a specific set of metadata

Besides having properties a WordPress page has a key/value collection of custom meta data. My other type of facet adds conditions to this collection:

class Meta_Facet extends Facet
    /* (non-PHPdoc)
    * @see DVM_Facets_Facet::add_facet()
    public function add_facet(&$wp_query)
        $meta_query = $wp_query->get('meta_query');
        $meta_query[] =
                'key' => $this->key,
                'value' => $this->value,
                'compare' => 'LIKE'
        $wp_query->set('meta_query', $meta_query);

I use this type to create a facet that show only pages with a specific template:

$d = new Meta_Facet();
$d->key = '_wp_page_template';
$d->value = 'article.php';

Pages that descend from a specific page

The last type of facet shows only descendants of a specific page. I use this kind of facets to show results from different sections of my page.

To do this I need to get a list of all the descendants ids. Then I add this list to the query variable post__in. This narrows down the search to only the list of descendants.

So a ancestor facet is much like a property facet – we just need the list of ids. So I extent the property facet and inject a data access object that can create the list:

class Ancestor_Facet extends Property_Facet
    public $ancestor_id;
    public $dal;
    * Constructs a new facet
    * @param int $id Facet id
    * @param string $name Facet name
    * @param string $icon Facet icon
    * @param int $ancestor_id Ancestor post id
    * @param DAL $dal Data access layer
    function __construct($id, $name, $icon, $ancestor_id, $dal)
        $this->ancestor_id = $ancestor_id;
        $this->dal = $dal;
        parent::__construct($id, $name, $icon, 'post__in', '');

    /* (non-PHPdoc)
    * @see Property_Facet::add_facet()
    public function add_facet(&$wp_query)
        $this->value =

Applying the facet

The next part of the puzzle is the mechanism that add the facet to the search when the request /s=foobar&facet_id=4 is made. This is done by the Applier with is hooked up to the pre_get_posts event and hence run each time the wp_query is about to fetch pages.

class Applier
    * Applies any selected facets to the wp_query
    * @param WP_Query $wp_query
    function on_pre_get_posts(&$wp_query)
        if ($wp_query->is_search())
            $facet_query = new DVM_Facets_Facet_Query();
            if ($facet_query->has_facet_id())
                $dal = new DVM_Facets_Dal();
                $facet = $dal->

The wp_query is used extensively in WordPress but most of the time the applier does not do anything either because the wp_query is not prepared for a search or because no facets are specified in the query string.

But when a search is done and a facet is requested the applier gets the facet and then add it to the wp_query.

The filter uses a Facet_Query to interact with the query string. This object encapsulates the query string and the parsing of it.