cannot override post_types in WP_Query()

Question

I'm creating a featured post slider which takes 4 pages or posts from the 'featured' category. I'm trying to use WP_Query, but am having no joy :(

my slider query is:

$ml_featured = new WP_Query( 

    array(
        'posts_per_page' => 4,
        'post_type' => array('page', 'post'),
        'category__in' => 22
    )
);


if( $ml_featured->have_posts() ) {
    while( have_posts() ) : $ml_featured->the_post();

        // looping

    endwhile;
}
else {
    // no posts
}

fire_PHP shows the WP_Query contains:

WP_Query(
    query =
    array(
        ['posts_per_page'] => 4
        ['post_type'] =>
            array(
                [0] => 'page'
                [1] => 'post'
            )
        ['category__in'] => 22
    )

    query_vars =
    array(

        ['posts_per_page'] => 4
        ['post_type'] =>
            array(
                [0] => 'wod'
                [1] => 'attachment'
            )
        ['category__in'] =>
            array(
                [0] => 22
            ) 

I'm using Genesis, building upon the sample child theme. In order to display my custom post type 'WOD' on the homepage I've added to functions.php:

add_filter( 'pre_get_posts', 'ml_wods_home' );
function ml_wods_home( $query ) {

    if ( is_home() && false == $query->query_vars['suppress_filters'] )
        $query->set( 'post_type', array( 'wod', 'attachment' ) );

    return $query;
}

as suggested at Justin Tadlock. This works well on its own, but seems to be interfering with the slider query:

Please note that query['post_type'] is overwritten by query_vars['post_type']. In addition, the browser is timing out frequently as I mess around...

I found that Michael Fields had a related issue , posted a solution, (which I think I'm implementing) but its not working for me, and I don't understand it..

What am I missing?! TIA, Tim

Answer 1

First of all,while( have_posts()): should be while( $ml_featured->have_posts()):

And secondly, in your pre_get_posts callback you check:

false == $query->query_vars['suppress_filters']

But this returns true for every WP_Query query and any get_posts query where 'supress_filters' is explicitly set to false. If you want to check if the query is the 'main' query (i.e. query associated with the global $wp_query), you can use:

$query->is_main_query();

(see documentation here). This returns true if $query is the main query and false otherwise. This function was introduced in 3.3. Before 3.3 you can probably check (untested):

global $wp_the_query;
if($query === $wp_the_query){
  //is main query
}else{
  // is not main query
}

Alternatively (but preferably use the methods above), to alter the main query only:

 global $wp_query;
 $wp_query->set( 'post_type', array( 'wod', 'attachment' ) );