WP_Query get other table column values - wordpress

Question

I am trying to fetch another table column by WP_Query but it does not work.
Note: I can easily do this by $wpdb->get_results( 'custom_sql' ) but I want to do it by WP_Query
My code
function join_my_query( $join, $wp_query ) {
global $wpdb;
if ( $wp_query->get( 'custom_logic' ) === true ) {
$join .= " LEFT JOIN $wpdb->postmeta as pm ON $wpdb->posts.ID = pm.post_id ";
}
return $join;
}
add_filter( 'posts_join', 'join_my_query', 10, 2 );
$query = new WP_Query(
array(
'custom_logic' => true,
'post_type' => 'post',
)
);
It's giving me SQL like below, which does not select any column from the joined table!
Generated SQL
SELECT
SQL_CALC_FOUND_ROWS wp_posts.ID
FROM
wp_posts
LEFT JOIN wp_postmeta as pm ON wp_posts.ID = pm.post_id
WHERE
1 = 1
AND (
(
wp_posts.post_type = 'post'
AND (
wp_posts.post_status = 'publish'
OR wp_posts.post_status = 'future'
OR wp_posts.post_status = 'draft'
OR wp_posts.post_status = 'pending'
)
)
)
ORDER BY
wp_posts.post_date DESC
LIMIT
0, 10
It does not select any column from my joined table. I have use var_dump($query->posts[0]) which return a WP_Post object and it has no column from my joined table. How I can get columns from my joined table with WP_Query like below
My Goal
$post = $query->posts[0];
$post->meta_key; // from my joined table, not work
$post->pm->meta_key; // pm is alias used in join, still not work

Answer 1

According to your expected result, the posts_join filter is not required. You can do this with the posts_clauses filter easily!
add_filter( 'posts_clauses', 'modify_post_clauses_callback' , 10, 2 );
function modify_post_clauses_callback( $clauses, $wp_query ) {
if ( $wp_query->get( 'custom_logic' ) === true ) {
global $wpdb;
$clauses['fields'] = $clauses['fields'] . ', pm.meta_key';
$clauses['join'] = "LEFT JOIN {$wpdb->postmeta} as pm ON {$wpdb->posts}.ID = pm.post_id";
}
return $clauses;
}
$query = new WP_Query(
array(
'custom_logic' => true,
'post_type' => 'post',
)
);
$post = $query->posts[0];
$post->meta_key;