Update Manage Stock - Grouped products

matt.php

/**
*   Performs check before saving/inserting products
*/
function grouped_product_check($post_id, $post_data) {
    // If this is just a revision, don't do anything.
    if (wp_is_post_revision($post_id))
        return;

    if ($post_data['post_type'] == 'product') {
	    if( $post_data['post_parent'] != 0 ) { 
	        update_post_meta($post_id, '_manage_stock', 'yes');
	    }
    }
}

add_action( 'pre_post_update', 'grouped_product_check', 10, 2);