WordPress Custom Fields Won't Save

Question

I've created two custom post types, buildings and availabilities, and added a query on availabilities to select what building it belongs too but I can't seem to get it to save.

add_action('admin_init', 'p2p2_add_building_metabox');

function p2p2_add_building_metabox(){
add_meta_box( 
    'building_availability', 
    __('Building', 'bandpress'), 
    'p2p2_building_availability_metabox', 
    'availabilities', 
    'side', 
    'default', 
    array( 'id' => 'p2p2_building') 
);}

function p2p2_building_availability_metabox($post, $args) {
    wp_nonce_field( plugin_basename( __FILE__ ), 'p2p2_building_nonce' );
    $building_id = get_post_meta($post->ID, 'p2p2_building', true);

    echo "<p>Select The Building of This Availability</p>";
    echo "<select id='p2p2_building' name='p2p2_building'>";

    // Query the buildings here
    $query = new WP_Query( 'post_type=building' );
    while ( $query->have_posts() ) {
        $query->the_post();
        $id = get_the_ID();
        $selected = "";

        if ($id == $author_id) {
            $selected = ' selected="selected"';
        }
        echo '<option' . $selected . ' value=' . $id . '>' . get_the_title() . '</option>';
    }

    echo "</select>";
}
 
add_action('save_post', 'p2p2_save_building_metabox', 1, 2);
function p2p2_save_building_metabox($post_id, $post){
    // Don't wanna save this now, right?
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return;
    
    if ( !isset( $_POST['p2p2_building_nonce'] ) )
        return;

    if ( !wp_verify_nonce( $_POST['p2p2_building_nonce'], plugin_basename( __FILE__ ) ) )
        return;

    // We do want to save? Ok!
    $key = 'p2p2_building';
    $value = $_POST["p2p2_building"];
    if ( get_post_meta( $post->ID, $key, FALSE ) ) { // If the custom field already has a value
        update_post_meta( $post->ID, $key, $value );
    } else { // If the custom field doesn't have a value
        add_post_meta( $post->ID, $key, $value );
    }

    if ( !$value )
        delete_post_meta( $post->ID, $key ); // Delete if blank
}

Answer 1

It seems you have one error in your code...

if($id == $author_id){
    $selected = ' selected="selected"';
}

It should be $building_id instead of $author_id.
I tested the rest of the code and the saving is working fine.

If you had printed/echoed out the value of this line ...

$building_id = get_post_meta($post->ID, 'p2p2_building', true);

like print_r($building_id);, you would had seen, that the correct post-ID was already saved. So only your select-element was not able to set the "selected" attribute correctly.

P.S. Maybe you also want to add an empty default option to the select field. As sometimes a user wont fill out this field, and than the first options post-ID gets always saved.

Something like...

echo "<select id='p2p2_building' name='p2p2_building'>";
// empty default option:
echo "<option value="">Select building...</option>";