Change template on the fly based on post parent selection

Question

I want to change my page template automatically when someone selects a specific post parent.

I have the code to update the template but couldn't find anything for detection of post parent attribute changes. Can you help?

Answer 1

This is how you do it...

function update_post_template($post_id, $post){

    //set $post_parent_id however you wish  

    if ($post->post_parent == $post_parent_id) {

        //unhook save_post action to avoid infinite loop
        remove_action('save_post', 'update_post_template'); 

        //change template
        update_post_meta( $post_id, '_wp_page_template', 'custom_template.php' );

        //re-hook save_post action
        add_action('save_post', 'update_post_template');

    }
}

add_action('save_post', 'update_post_template', 10, 2);

Notes:

To explain what is happening, first we hook onto the save_post action which is fired when inserting/updating a post. We then check for the existence of a certain post_parent id which is up to you to decide and determine which (you may want to extend this conditional statement to cover multiple cases). If we find a match, we unhook the save_post action to avoid creating an infinite loop when we call our update_post_meta function. Once the page template has been updated, we re-hook the save_post action and continue on our merry way.

Answer 2

Using MySQL you can do;

$result= $wpdb->get_row("select post_parent from wp_posts where ID=".$post->ID);

if(intval($result)>0)
{
     //your logic
}
else
{
     //Do something else
}