How do I save each option in a multiple select menu as it's own meta_key + meta_value pair?

Question

How do I save each option in a multiple select menu as it's own meta_key + meta_value pair?

This is the basic multiple select menu. Note: multiple select menus allow you to select more than one option.

<select name="products[]" multiple>
    <option value="1">Product One</option>
    <option value="2">Product Two</option>
    <option value="3">Product Three</option>
    .... etc
</select>

The bit of code inside the save_post function that I need help with.

if ( isset( $_POST[ 'products' ] ) ) {
    foreach ( $_POST[ 'products' ] as $product ) {
        // this would just keep adding the products as separate meta keys, which is what I want, but I need to modify it so it deletes them when they are unselected.
        add_post_meta( $post_id, 'products', $product, false );
    }   
}

I'd like each option to be it's own meta key and value pair and not a serialized array. If the multiple select menu is updated I'd like old values no longer selected to be removed from the postmeta table with delete_post_meta.

For example, the wp_postmeta table would look like this:

post_id     meta_key             meta_value
45          products             2
63          products             3
12          products             1

Answer 1

Try this inside save_post but please note the code is not tested

$old = get_post_meta($post_id, 'products');
$new = isset ( $_POST['products'] )  ? $_POST['products'] : array();
if ( empty ($new) ) {
   // no products selected: completely delete alla meta values for the post
   delete_post_meta($post_id, 'products');
} else {
  $already = array();
  if ( ! empty($old) ) {
    foreach ($old as $value) {
      if ( ! in_array($value, $new) ) {
        // this value was selected, but now it isn't so delete it
        delete_post_meta($post_id, 'products', $value);
      } else {
        // this value already saved, we can skip it from saving
        $already[] = $value;
      }
    }
  }
  // we don't save what already saved
  $to_save = array_diff($new, $already);
  if ( ! empty($to_save) ) {
    foreach ( $to_save as $product ) {
       add_post_meta( $post_id, 'products', $product);
    }
  }
}