how to save multiple checkboxes value in wordpress dynamically

Question

I already read this question but it doesn't help me. I create metabox to connect 2 custom post type( 'author' and 'book' ) so that when user want to add a new book can choose author from metabox.

when I want to save authors of the book to db just the last author stores in db (wp_postmeta table).

here is my code

my metabox

add_action('admin_init', 'p2p2_add_author_metabox');

add_action('admin_init', 'p2p2_add_author_metabox');

function p2p2_add_author_metabox(){
    add_meta_box( 
        'book_author', 
        __('نویسنده کتاب', 'bandpress'), 
        'p2p2_book_author_metabox', 
        'book', 
        'side', 
        'default', 
        array( 'id' => 'p2p2_author') 
    );
}

show in book custom post type

function p2p2_book_author_metabox($post, $args) {
 wp_nonce_field( plugin_basename( __FILE__ ), 'p2p2_book_author_nonce' );
 $author_id = get_post_meta($post->ID, 'p2p2_book_author', true);
 echo "<p>انتخاب نویسنده کتاب</p>";
  // Query the authors here
  $query = new WP_Query( 'post_type=author' );
   while ( $query->have_posts() ) {
        $query->the_post();
        $id = get_the_ID();
        $input_check = "";
        $cooaprate=get_field('auth-trans'); 
       if(($cooaprate[0]=='نویسنده')){



         $input_check="checked($author_id[$id][0],'yes')";
          echo '<input name="save-author-to-book[]" type="checkbox"' . $input_check . ' value=' . $id ; 

            if (get_post_meta( $post_id, $key,$id )) {
            echo " checked";

          }


          echo '>' . get_the_title() . '</br>';







        }

}
}

save in database

add_action( 'save_post', 'p2p2_save_author_metabox' );
    function p2p2_save_author_metabox( $post_id) {
       if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return;
    if ( !isset( $_POST['p2p2_book_author_nonce'] ) )
        return;
    if ( !wp_verify_nonce( $_POST['p2p2_book_author_nonce'], plugin_basename( __FILE__ ) ) )
        return;
    if (isset($_POST['save-author-to-book'])) {




    $key = 'save-author-to-book';
    $values = $_POST["save-author-to-book"];

    foreach ($values as $value) {


    if ( get_post_meta( $post_id, $key, true ) ) { // 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
}
 }


}

I know the problem is come from if ( get_post_meta( $post_id, $key, true ) ) it overwrite same records but I really don't know how can I fix it.

any idea would be appreciated.

Answer 1

Here you can just save the info in array and save that array in your post meta.

some thing like this will work;

$key = 'save-author-to-book';
$values_to_save = array();
$new_values = $_POST["save-author-to-book"];
$existing_values = get_post_meta( $post_id, $key, true ) ;

if(!empty($existing_values)){
   foreach($existing_values as $existing_value){
      $values_to_save[] = $existing_value;
   }
}


if(!empty($new_values)){
   foreach($new_values as $new_value ){
      $values_to_save[] = $new_value ;
   }
}

 update_post_meta( $post_id, $key, $values_to_save ); // you don't need to use if condition as  update post meta will take care

And here is how you can display the information stored in an array;

$key = 'save-author-to-book';
$values = get_post_meta( $post_id, $key, true );

foreach($values as $value){
   echo $value . '<br>';
}