How to amend wordpress search so it queries taxonomy terms and category terms?

Question

I want to amend the search so if say I have taxonomy called "author" and add a term of "Ian Rankin" to a post, if I search for "Ian Rankin", I want that post to come up. I guess at the moment it only searches titles and content. How can I make it search terms too?

Answer 1

You can alter the search query using filter hooks to join the taxonomy tables.

e.g. to also search on the 'author' taxonomy

First join the taxonomy tables

function tax_search_join( $join )
{
  global $wpdb;
  if( is_search() )
  {
    $join .= "
        INNER JOIN
          {$wpdb->term_relationships} ON {$wpdb->posts}.ID = {$wpdb->term_relationships}.object_id
        INNER JOIN
          {$wpdb->term_taxonomy} ON {$wpdb->term_taxonomy}.term_taxonomy_id = {$wpdb->term_relationships}.term_taxonomy_id
        INNER JOIN
          {$wpdb->terms} ON {$wpdb->terms}.term_id = {$wpdb->term_taxonomy}.term_id
      ";
  }
  return $join;
}
add_filter('posts_join', 'tax_search_join');

then find the search term in the taxonomy 'author'

function tax_search_where( $where )
{
  global $wpdb;
  if( is_search() )
  {
    // add the search term to the query
    $where .= " OR
    (
      {$wpdb->term_taxonomy}.taxonomy LIKE 'author'
      AND
      {$wpdb->terms}.name LIKE ('%".$wpdb->escape( get_query_var('s') )."%')
    ) ";
  }
  return $where;
}
add_filter('posts_where', 'tax_search_where');

and finally group the results by post id to avoid duplicate results because of the join

function tax_search_groupby( $groupby )
{
  global $wpdb;
  if( is_search() )
  {
    $groupby = "{$wpdb->posts}.ID";
  }
  return $groupby;
}
add_filter('posts_groupby', 'tax_search_groupby');

Answer 2

It could also be done by using MySQL's EXISTS function with a subquery that joins term_relationships with terms. I have created the following snippet and put it into functions.php. This applies to at least WordPress Version 5.3.1:

// Search titles and tags
function and_extend_search( $search, &$wp_query ) {

    global $wpdb;

    if ( empty( $search ))
        return $search;

    $terms = $wp_query->query_vars[ 's' ];
    $exploded = explode( ' ', $terms );
    if( $exploded === FALSE || count( $exploded ) == 0 )
        $exploded = array( 0 => $terms );

    $search = '';
    
    foreach( $exploded as $tag ) {
        $search .= " AND (
            ($wpdb->posts.post_title LIKE '%$tag%')

            OR EXISTS
            (
                SELECT 
                    *
                FROM 
                    $wpdb->term_relationships 
                LEFT JOIN 
                    $wpdb->terms 
                ON 
                    $wpdb->term_relationships.term_taxonomy_id = $wpdb->terms.term_id
                WHERE
                        $wpdb->terms.name LIKE '%$tag%'
                    AND
                        $wpdb->term_relationships.object_id = $wpdb->posts.ID
            )
        )";
    }

    return $search;
}
add_filter('posts_search', 'and_extend_search', 500, 2);