wp_ajax_add_meta()

function wp_ajax_add_meta() {
	check_ajax_referer( 'add-meta', '_ajax_nonce-add-meta' );
	$c = 0;
	$pid = (int) $_POST['post_id'];
	$post = get_post( $pid );

	if ( isset($_POST['metakeyselect']) || isset($_POST['metakeyinput']) ) {
		if ( !current_user_can( 'edit_post', $pid ) )
			wp_die( -1 );
		if ( isset($_POST['metakeyselect']) && '#NONE#' == $_POST['metakeyselect'] && empty($_POST['metakeyinput']) )
			wp_die( 1 );

		// If the post is an autodraft, save the post as a draft and then attempt to save the meta.
		if ( $post->post_status == 'auto-draft' ) {
			$post_data = array();
			$post_data['action'] = 'draft'; // Warning fix
			$post_data['post_ID'] = $pid;
			$post_data['post_type'] = $post->post_type;
			$post_data['post_status'] = 'draft';
			$now = current_time('timestamp', 1);
			/* translators: 1: Post creation date, 2: Post creation time */
			$post_data['post_title'] = sprintf( __( 'Draft created on %1$s at %2$s' ), date( __( 'F j, Y' ), $now ), date( __( 'g:i a' ), $now ) );

			$pid = edit_post( $post_data );
			if ( $pid ) {
				if ( is_wp_error( $pid ) ) {
					$x = new WP_Ajax_Response( array(
						'what' => 'meta',
						'data' => $pid
					) );
					$x->send();
				}

				if ( !$mid = add_meta( $pid ) )
					wp_die( __( 'Please provide a custom field value.' ) );
			} else {
				wp_die( 0 );
			}
		} elseif ( ! $mid = add_meta( $pid ) ) {
			wp_die( __( 'Please provide a custom field value.' ) );
		}

		$meta = get_metadata_by_mid( 'post', $mid );
		$pid = (int) $meta->post_id;
		$meta = get_object_vars( $meta );
		$x = new WP_Ajax_Response( array(
			'what' => 'meta',
			'id' => $mid,
			'data' => _list_meta_row( $meta, $c ),
			'position' => 1,
			'supplemental' => array('postid' => $pid)
		) );
	} else { // Update?
		$mid = (int) key( $_POST['meta'] );
		$key = wp_unslash( $_POST['meta'][$mid]['key'] );
		$value = wp_unslash( $_POST['meta'][$mid]['value'] );
		if ( '' == trim($key) )
			wp_die( __( 'Please provide a custom field name.' ) );
		if ( '' == trim($value) )
			wp_die( __( 'Please provide a custom field value.' ) );
		if ( ! $meta = get_metadata_by_mid( 'post', $mid ) )
			wp_die( 0 ); // if meta doesn't exist
		if ( is_protected_meta( $meta->meta_key, 'post' ) || is_protected_meta( $key, 'post' ) ||
			! current_user_can( 'edit_post_meta', $meta->post_id, $meta->meta_key ) ||
			! current_user_can( 'edit_post_meta', $meta->post_id, $key ) )
			wp_die( -1 );
		if ( $meta->meta_value != $value || $meta->meta_key != $key ) {
			if ( !$u = update_metadata_by_mid( 'post', $mid, $value, $key ) )
				wp_die( 0 ); // We know meta exists; we also know it's unchanged (or DB error, in which case there are bigger problems).
		}

		$x = new WP_Ajax_Response( array(
			'what' => 'meta',
			'id' => $mid, 'old_id' => $mid,
			'data' => _list_meta_row( array(
				'meta_key' => $key,
				'meta_value' => $value,
				'meta_id' => $mid
			), $c ),
			'position' => 0,
			'supplemental' => array('postid' => $meta->post_id)
		) );
	}
	$x->send();
}