我找到了一种方法来实现这一点。我的想法是对的pre_get_posts 就是要走的路。首先，我添加了一个操作：

// Filter the search page
add_filter(\'pre_get_posts\', \'search_pre_get_posts\');

然后我添加了函数。此函数首先检查上下文是否正确。在自定义搜索表单中，我使用nonce。nonce帮助我验证正在运行的是我的自定义搜索，而不是内置搜索。然后，我操作将发送到查询的查询变量。我在这里展示了我的全部功能，其中包括所有变量的验证和清理，以及如何操作查询：

/**
 * Filters for search of events
 *
 * When a search is initiated, this filter will check to see if it is an
 * event search. If so, it will process the search data associated with
 * event searches and add it to the $query object prior to it being used
 * to query the database.
 *
 * @param $query The query object generated for this page
 * @return void
 */
function search_pre_get_posts($query)
{
    // Verify that we are on the search page that that this came from the event search form
    if($query->query_vars[\'s\'] != \'\' && is_search() && isset($_GET[\'prefix-submitted\']) && $_GET[\'prefix-submitted\'] && wp_verify_nonce($_GET[\'prefix-search-nonce\'], \'prefix-search\'))
    {
        // Always order by date
        $query->set(\'orderby\', \'meta_value\');
        $query->set(\'order\', \'ASC\');
        $query->set(\'meta_key\', \'_datetime-from\');

        // Set the text string for the search
        $clean_text = (isset($_GET[\'prefix-text\'])) ? wp_strip_all_tags($_GET[\'prefix-text\']) : \'\';
        $query->set(\'s\', $clean_text);

        // Validate dates
        $clean_from = (isset($_GET[\'prefix-from\'])) ? $this->handle_date($_GET[\'prefix-from\']) : false;
        $clean_to = (isset($_GET[\'prefix-to\'])) ? $this->handle_date($_GET[\'prefix-to\']) : false;

        // Validate taxonomies
        $clean_type = (isset($_GET[\'prefix-type\'])) ? $this->handle_taxonomy_id($_GET[\'prefix-type\'], \'prefix-type\') : false;
        $clean_location = (isset($_GET[\'prefix-location\'])) ? $this->handle_taxonomy_id($_GET[\'prefix-location\'], \'prefix-location\') : false;

        // Build meta_query variable based on date pieces
        if($clean_from && $clean_to)
        {
            $meta_query = array(
                array(
                    \'key\' => \'_datetime-from\',
                    \'value\' => array($clean_from, $clean_to),
                    \'compare\' => \'BETWEEN\',
                    \'type\' => \'DATE\'
                )
            );
        }
        elseif($clean_from)
        {
            $meta_query = array(
                array(
                    \'key\' => \'_datetime-from\',
                    \'value\' => $clean_from,
                    \'compare\' => \'>=\',
                    \'type\' => \'DATE\'
                )
            );
        }
        elseif($clean_to)
        {
            $meta_query = array(
                array(
                    \'key\' => \'_datetime-from\',
                    \'value\' => $clean_from,
                    \'compare\' => \'>=\',
                    \'type\' => \'DATE\'
                )
            );
        }
        else
            $meta_query = \'\';

        // Set the meta query
        $query->set(\'meta_query\', $meta_query);

        // Build tax query based on taxonomy pieces
        if($clean_type && $clean_location)
        {
            $tax_query = array(
                \'relation\' => \'AND\',
                array(
                    \'taxonomy\' => \'prefix-type\',
                    \'field\' => \'id\',
                    \'terms\' => $clean_type,
                    \'operator\' => \'IN\'
                ),
                array(
                    \'taxonomy\' => \'prefix-location\',
                    \'field\' => \'id\',
                    \'terms\' => $clean_location,
                    \'operator\' => \'IN\'
                )
            );
        }
        elseif($clean_type)
        {
            $tax_query = array(
                array(
                    \'taxonomy\' => \'prefix-type\',
                    \'field\' => \'id\',
                    \'terms\' => $clean_type,
                    \'operator\' => \'IN\'
                )
            );
        }
        elseif($clean_location)
        {
            $tax_query = array(
                array(
                    \'taxonomy\' => \'prefix-location\',
                    \'field\' => \'id\',
                    \'terms\' => $clean_location,
                    \'operator\' => \'IN\'
                )
            );
        }
        else
            $tax_query = \'\';

        // Set the tax query
        $query->set(\'tax_query\', $tax_query);
    }
}