wordpress nested loop

Question

I am building nested loop query.I have meta box that capture post id (sorter and repeater).so far my code

while ( have_posts() ) : the_post();
    ?>
<div id="<?php echo $post->post_name; ?>">
    <h1><?php the_title(); ?></h1>
    <?php
    the_content();


    $urls = get_post_meta( get_the_ID(), 'mymeta_url_group', false );

    foreach ( $urls as $url ) {
        //echo $url["mymeta_url"];
        foreach ( $url['mymeta_slider'] as $u ) {
            //echo "$u<br> "; shows post id like 67 68 56 etc

            $my_query = new WP_Query( array( 'post_type' => 'slider', 'p' => "'.$u  .'" ) );
            if ( $my_query->have_posts() ) {
                while ( $my_query->have_posts() ) {
                    $my_query->the_post();
                    the_title();
                }
            }
            wp_reset_postdata();
        }

but my output result got duplicated.that is post type slider has 3 posts query shows 6 posts duplicating twice.

Answer 1

Do I see that right, that you are doing a WP_Query for every single post, you want to retrieve? That is highly inefficient, you can use the post__in argument supplied by WP_Query. (http://codex.wordpress.org/Class_Reference/WP_Query#Post_.26_Page_Parameters)

foreach ( $urls as $url ) {
    //echo $url["mymeta_url"];
    foreach ( $url['mymeta_slider'] as $u ) {
        //echo "$u<br> "; shows post id like 67 68 56 etc
        $posts_to_recive[] = $u;
    }
    $my_query = new WP_Query( array( 'post_type' => 'slider', 'post__in' => $posts_to_recive ) );

    if ( $my_query->have_posts() ) {
       while ( $my_query->have_posts() ) {
            $my_query->the_post();
            the_title();
       }
    }

    wp_reset_postdata();
}

And yet this is highly inefficient. Keep the amount of WP_Querys as low as possible.