WP_Tax_Query with post_tag not working

Question

Using the pre_get_posts hook, I'm trying to allow users to narrow down their search by limiting it to posts with certain (multiple) tags applied.

This is my hook and function:

add_filter('pre_get_posts', array($this, 'pre_get_posts'), 0);

function pre_get_posts( $query ) {
if ( !is_admin() && $query->is_search ) {
    $query->set( 'post_type', array( 'post', 'resource', 'initiative') );
    if (isset($_GET['tags'])) {
        $tax_query = array(
            array(
            'taxonomy' => 'post_tag',
            'field' => 'slug',
            'terms' => $_GET['tags'],
            'operator' => 'AND',
            )
        );
        $tax_query = new WP_Tax_Query($tax_query);
        $query->set('tax_query', $tax_query);
    }
}
return $query;

}

As you can see, I'm also expanding the search to include two custom post types. In the results of a search, the custom post types are added correctly and I can see them in the query that's generated, so I know that the hook and function are being called correctly. The post_tag part of the query is not added though.

The WP_Query Object's query_vars element correctly (I think) contains the tax_query, but the WP_Query Object contains its own tax_query which is empty, and the SQL in the request also doesn't have it. Here's a (slightly stripped-down) var_dump of the WP_Query Object:

WP_Query Object (
[query_vars] => Array
    (
        [s] => ecosystem

        [post_type] => Array
            (
                [0] => post
                [1] => resource
                [2] => initiative
            )

        [tax_query] => WP_Tax_Query Object
            (
                [queries] => Array
                    (
                        [0] => Array
                            (
                                [taxonomy] => post_tag
                                [terms] => Array
                                    (
                                        [0] => culture-change
                                        [1] => peace
                                    )

                                [include_children] => 1
                                [field] => slug
                                [operator] => AND
                            )

                    )

                [relation] => AND
            )
        [search_terms_count] => 1
        [search_terms] => Array
            (
                [0] => ecosystem
            )

        [search_orderby_title] => Array
            (
                [0] => jcgy_posts.post_title LIKE '%ecosystem%'
            )

        [order] => DESC
    )

[tax_query] => WP_Tax_Query Object
    (
        [queries] => Array
            (
            )

        [relation] => AND
    )

[query] => Array
    (
        [s] => ecosystem
    )

[request] => SELECT SQL_CALC_FOUND_ROWS DISTINCT jcgy_posts.* FROM jcgy_posts  LEFT JOIN jcgy_postmeta as jcgy_postmeta on jcgy_posts.ID = jcgy_postmeta.post_id AND jcgy_postmeta.meta_key = '_EventStartDate' WHERE 1=1  AND (((jcgy_posts.post_title LIKE '%ecosystem%') OR (jcgy_posts.post_content LIKE '%ecosystem%')))  AND jcgy_posts.post_type IN ('post', 'resource', 'initiative') AND (jcgy_posts.post_status = 'publish' OR jcgy_posts.post_author = 1 AND jcgy_posts.post_status = 'private')  ORDER BY jcgy_posts.post_title LIKE '%ecosystem%' DESC, post_date DESC LIMIT 0, 10

All the research I've done tells me that I've structured my addition to the query correctly, yet clearly something isn't working. I am tearing my hair out. Any ideas?

Answer 1

I did get the thing working, though I'm not entirely sure how.

I made two changes: Removed the "convert array to Tax_Query object" step (which I only put in because using an array wasn't working before) Added a direct assignment of the array to the $query object (rather than just using the set() method).

My final (working) function is:

function pre_get_posts( $query ) {
if ( !is_admin() && $query->is_search ) {

    if (isset($_GET['post_types'])) {
        $query->set( 'post_type', $_GET['post_types']);
    } else {
        $query->set( 'post_type', array( 'post', 'resource', 'initiative') );
    }
    if (isset($_GET['tags'])) {
        $tax_query = array(
            array(
            'taxonomy' => 'post_tag',
            'field' => 'slug',
            'terms' => $_GET['tags'],
            'operator' => 'AND',
            )
        );
        $query->set('tax_query', $tax_query); // do I need both of these lines?
        $query->tax_query = $tax_query;       // let's leave them both in for safety
    }
}
return $query;

}