Save custom dropdown field.

Hey there!!

Well, I have a little problem with my custom meta box. I create a custom post which includes a custom meta field dropdown. The dropdown displays the title of the posts from another custom posts that I want to link with when save. But when I clic on publish the dropdown doesn´t save.

Here is my code.

Here I create my custom post:

function my_custom_post_category_product() {
	$labels = array(
		'name'               => _x( 'Categories Products', 'post type general name' ),
		'singular_name'      => _x( 'Categories Products', 'post type singular name' ),
		'add_new'            => _x( 'Add New' ),
		'add_new_item'       => __( 'Add New Categories Products' ),
		'edit_item'          => __( 'Edit Categories Products' ),
		'new_item'           => __( 'New Categories Products' ),
		'all_items'          => __( 'All Categories Products' ),
		'view_item'          => __( 'View Categories Products' ),
		'search_items'       => __( 'Search Categories Products' ),
		'not_found_in_trash' => __( 'No Categories Products found in the Trash' ),
		'not_found'          => __( 'No Categories Products found' ),
		'parent_item_colon'  => '',
		'menu_name'          => 'Categoria Producto'
	);
	$args = array(
		'labels'        => $labels,
		'description'   => 'Holds our categories specific data',
		'public'        => true,
		'menu_position' => 100,
		'supports'      => array( 'title', 'editor', 'thumbnail', 'excerpt', 'comments' ),
		'has_archive'   => true,
		'taxonomies' => array('category'),
	);
	register_post_type( 'category_product', $args );
}
add_action( 'init', 'my_custom_post_category_product' );

function my_updated_messages_category_product( $messages ) {
	global $post, $post_ID;
	$messages['categories'] = array(
		0 => '',
		1 => sprintf( __('Categories Products updated. <a href="%s">View Categories Products</a>'), esc_url( get_permalink($post_ID) ) ),
		2 => __('Custom field updated.'),
		3 => __('Custom field deleted.'),
		4 => __('Categories updated.'),
		5 => isset($_GET['revision']) ? sprintf( __('Categories Products restored to revision from %s'), wp_post_revision_title( (int) $_GET['revision'], false ) ) : false,
		6 => sprintf( __('Categories Products published. <a href="%s">View Categories Products</a>'), esc_url( get_permalink($post_ID) ) ),
		7 => __('Categories Products saved.'),
		8 => sprintf( __('Categories Products submitted. <a target="_blank" href="%s">Preview Categories Products</a>'), esc_url( add_query_arg( 'preview', 'true', get_permalink($post_ID) ) ) ),
		9 => sprintf( __('Categories Products scheduled for: <strong>%1$s</strong>. <a target="_blank" href="%2$s">Preview Categories Products</a>'), date_i18n( __( 'M j, Y @ G:i' ), strtotime( $post->post_date ) ), esc_url( get_permalink($post_ID) ) ),
		10 => sprintf( __('Categories Products draft updated. <a target="_blank" href="%s">Preview Categories Products</a>'), esc_url( add_query_arg( 'preview', 'true', get_permalink($post_ID) ) ) ),
	);
	return $messages;
}
add_filter( 'post_updated_messages', 'my_updated_messages_category_product' );

And then the function to create the custom meta box and save the data:

add_action('add_meta_boxes', 'product_post_options_box',10,2);
/* saved the data */
add_action( 'save_post', 'save_product_post_options_box' );

function product_post_options_box($post_type, $post) {
    if ('category_product' == $post_type) //only add to your post type
        add_meta_box('post_info', 'Product Information', 'product_post_info', 'category_product', 'side', 'high');
}

function product_post_info() {
    global $post;
    /* List the posts*/
    $posts = get_posts( array(
        'post_type'      => array( 'categories' ),
        'posts_per_page' => -1
        )
    );
    //get saved data
    $saved = get_post_meta($post->ID,'_categories',true);
    if (empty($saved))
        $saved = array();
		//var_dump($saved);

    echo '<select name="categories" id="categories" multiple>'.
        '<option value="" selected="selected">Select a post</option>';
    foreach ($posts as $p) {
        $link = get_permalink( $p->ID);
        $selected = (in_array($link,$saved)) ? ' selected="selected"' : '';
        echo '<option'.$selected.' value="'.$link.'">'.get_the_title( $p->ID ).'</option>';
    }
    echo '</select>';

    // Use nonce for verification
    wp_nonce_field( plugin_basename( __FILE__ ), 'categories_nonce' );

}

function save_product_post_options_box($post_ID){
    // verify if this is an auto save routine.
    // If it is our form has not been submitted, so we dont want to do anything
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return;

    // First we need to check if the current user is authorised to do this action.
    if ( ! current_user_can( 'edit_post', $post_ID ) )
        return;

    // Secondly we need to check if the user intended to change this value.
    if ( ! isset( $_POST['categories_nonce'] ) || ! wp_verify_nonce( $_POST['categories_nonce'], plugin_basename( __FILE__ ) ) )
      return;

    // Thirdly check the post type
    if ('categories' != get_post_type($post_ID))
        return;

    $mydata =  $_POST['categories'];

    // Do something with $mydata
    // either using
    update_post_meta( $post_ID, '_category_product', $mydata, true);
}

At this point:

//get saved data
    $saved = get_post_meta($post->ID,'_categories',true);
    if (empty($saved))
        $saved = array();
		//var_dump($saved);

If I show $saved in order to see what it contains it shows the custom posts from WordPress. I have no idea where is the bug. Any idea?

Note: The dropdown must save multiple options (1 post or more)

Thanks a lot!