post__not_in不被排除在外

I am trying to exclude some posts with a custom meta_key within the hook pre_get_posts but for some reason is not working, the posts are not getting excluded. Taxonomies to be excluded work, but posts no.

add_action('pre_get_posts' , 'changeCourseCountry');
function changeCourseCountry($query){
    global $wpdb;
    $tax_query_merge = array(
        'relation' => 'AND',
        array(
            'taxonomy' => 'course_category',
            'field' => 'slug',
            'terms' => array('short-courses', 'mega-course'),
            'operator' => 'NOT IN'
        ),
    );
    $tax_query = array_merge($tax_query, $tax_query_merge);
    $exclude = $wpdb->get_col("SELECT post_id FROM $wpdb->postmeta WHERE meta_key = '_video_course'"); // here I get an array of posts
    $query->set('tax_query' ,$tax_query);
    $query->set('post__not_in', $exlcude);
    return $query
}

Please use the following instead of getting the column data,

$exclude = $wpdb->get_row("SELECT post_id FROM $wpdb->postmeta WHERE meta_key = '_video_course'"); // here I get an array of posts 

So it will be ,

add_action('pre_get_posts' , 'changeCourseCountry');
function changeCourseCountry($query){
    global $wpdb;
    $tax_query_merge = array(
        'relation' => 'AND',
        array(
            'taxonomy' => 'course_category',
            'field' => 'slug',
            'terms' => array('short-courses', 'mega-course'),
            'operator' => 'NOT IN'
        ),
    );
    $tax_query = array_merge($tax_query, $tax_query_merge);
    $exclude = $wpdb->get_row("SELECT post_id FROM $wpdb->postmeta WHERE meta_key = '_video_course'",ARRAY_A); // here I get an array of posts
    $query->set('tax_query' ,$tax_query);
    $query->set('post__not_in', $exlcude);
    return $query;
}

Also if you need multiple IDs then you need to use get_results and loop the array in foreach.

I also added ARRAY_A to $wpdb query to retrive the array.