Delete custom meta

I found a function to get the Meta ID by Meta Key. This allowed me to correctly assign my $meta_ID variable, and now the delete ajax button works perfectly.

No need to update the save function at all.

function get_mid_by_key( $post_id, $meta_key ) {
  global $wpdb;
  $mid = $wpdb->get_var( $wpdb->prepare("SELECT meta_id FROM $wpdb->postmeta WHERE post_id = %d AND meta_key = %s", $post_id, $meta_key) );
  if( $mid != '' )
    return (int)$mid;

  return false;
}

function wp_custom_attachment() {

wp_nonce_field(plugin_basename(__FILE__), 'wp_custom_attachment_nonce');

$html="<p class="description">";
    $html .= 'Upload your PDF here.';
$html .= '</p>';
$html .= '<input type="file" id="wp_custom_attachment" name="wp_custom_attachment" value="" size="25" />';

$pdf = get_post_meta(get_the_ID(), 'wp_custom_attachment', true);

if($pdf['url']){
    $html .=  '<p style="overflow-wrap: break-word;">Current file: ' .$pdf['url'] . '</p>';
    } 

$meta_id=get_mid_by_key( get_the_ID(), 'wp_custom_attachment' );

$delete_nonce = wp_create_nonce( 'delete-meta_' . $meta_id );
$html.=get_submit_button( __( 'Delete' ), "delete:wp_custom_attachment:meta-{$meta_id}::_ajax_nonce=$delete_nonce deletemeta", "deletemeta[{$meta_id}]", false, array( 'tabindex' => '6' ) );
$html.=wp_nonce_field( 'change-meta', '_ajax_nonce', false, false );

echo $html;

} // end wp_custom_attachment

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