public static function rebuild_activity_comment_tree( $parent_id, $left = 1 ) {
		global $wpdb;

		$bp = buddypress();

		// The right value of this node is the left value + 1.
		$right = intval( $left + 1 );

		// Get all descendants of this node.
		$comments    = BP_Activity_Activity::get_child_comments( $parent_id );
		$descendants = wp_list_pluck( $comments, 'id' );

		// Loop the descendants and recalculate the left and right values.
		foreach ( (array) $descendants as $descendant_id ) {
			$right = BP_Activity_Activity::rebuild_activity_comment_tree( $descendant_id, $right );
		}

		// We've got the left value, and now that we've processed the children
		// of this node we also know the right value.
		if ( 1 === $left ) {
			$wpdb->query( $wpdb->prepare( "UPDATE {$bp->activity->table_name} SET mptt_left = %d, mptt_right = %d WHERE id = %d", $left, $right, $parent_id ) );
		} else {
			$wpdb->query( $wpdb->prepare( "UPDATE {$bp->activity->table_name} SET mptt_left = %d, mptt_right = %d WHERE type = 'activity_comment' AND id = %d", $left, $right, $parent_id ) );
		}

		// Return the right value of this node + 1.
		return intval( $right + 1 );
	}