How to apply pre_get_posts to a custom query?

Question

How can I apply pre_get_posts to a custom query?

For example, if I have:

$custom_query = new WP_Query(...)
//code here

How can I apply the pre_get_posts for this $custom_query?

Answer 1

For some use cases, couldn't you use $query->get() to target specific custom queries?

For example: say you've got a custom query for a slider post-type. Perhaps something like this?

function wpse64950_filter_pre_get_posts( $query ) {
    if ( ! is_main_query() ) {
        if ( 'slider' == $query->get( 'post-type' ) ) {
            // This is a query of slider posts,
            // and isn't the main query;
            // Do something
        }
    }
    return;
}
add_filter( 'pre_get_posts', 'wpse64950_filter_pre_get_posts' );

Granted, this will target every such slider post-type custom query; so it won't work in every case. But it should work in some cases.

Answer 2

pre_get_posts fires for every post query.

There is really no reason to apply it to a specific custom query. If you don't want it to be applied to your custom query you need to make sure you are checking for is_main_query().