class.Gallery.php

<?php
new AssetGallery();

class AssetGallery {

	function __construct() {
		add_shortcode("latest_assets", array($this, 'shortcode'));
	}

	function shortcode($atts) {

			$a = shortcode_atts(array(
				'category' => null,
				'num' => 4,
				'columns' => 4,
			), $atts);

			$args = array(
				'post_type' => 'attachment',
				'post_mime_type' => 'image',
				'post_status' => 'any',
				'posts_per_page' => $a['num'],
			);
			if ($a['category'] !== null):
				switch($a['category']):
					case "cubitt":
					case "east":
					case "west":
					case "south":
					case "north":
					case "loveday":
					case "rhodes":
					case "connaught":
					// If the category is the name of a house. We assume we only use this on the house mini-sites.
					// Therefore we only get get only the photos that are marked as "display-on-homepage" as a tag in each mini-site.
						$args['tax_query'] = [
							[
								"taxonomy" => "post_tag",
								"field" => "slug",
								"terms" => "display-on-homepage"
							]
						];
					break;

					// For all other cases...
					default:
						$args['tax_query'] = array(
							"relation" => "OR",
							array(
								'taxonomy' => 'post_tag',
								'field' => 'slug',
								'terms' => $a['category'],
							),
							array(
								'taxonomy' => 'category',
								'field' => 'slug',
								'terms' => $a['category']
							)
						);
					break;
				endswitch;

			endif;

			$query = new WP_Query($args);

			$image_ids = array();

			foreach($query->posts as $post):
				$image_ids[] = $post->ID;
			endforeach;

			if (count($image_ids) == 0)
				return false;

			$list = implode(",", $image_ids);
			$return = "<div class=\"asset-gallery\">";
			$return .= do_shortcode("[gallery link=\"file\" columns=\"".$a['columns']."\" size=\"thumbnail\" orderby=\"rand\" ids=\"".$list."\"]");
			$return .= "</div>";

			wp_reset_postdata();
			wp_reset_query();

			return $return;
	}
}