Make WordPress look at specific post meta fields when doing a search

functions.php

<?php
add_action('posts_where_request', function($where) use($wpdb, $wp) {
  if(is_search()) {
		// Change this as needed
		$keys = implode("','", array('url', 'author', 'source', 'subtitle'));

		// Search in meta fields as well:
		$where .= " OR ({$wpdb->postmeta}.meta_key IN('{$keys}') AND {$wpdb->postmeta}.meta_value LIKE '%{$wp->query_vars['s']}%')";

		// Add in the postmeta table:
		add_filter('posts_join_request', function($join) use($wpdb) {
			return $join .= " LEFT JOIN {$wpdb->postmeta} ON {$wpdb->posts}.ID = {$wpdb->postmeta}.post_id ";
		});

		// No duplicate results
		add_filter('posts_distinct_request', function() {
			return 'DISTINCT';
		});
	}
	
	return $where;
});