BP_Document::get_folder_document_ids( bool $folder_id = false )

	public static function get_folder_document_ids( $folder_id = false ) {
		global $bp, $wpdb;

		if ( ! $folder_id ) {
			return false;
		}

		$folder_document_sql = $wpdb->prepare( "SELECT DISTINCT d.id FROM {$bp->document->table_name} d WHERE d.folder_id = %d", $folder_id );

		$cached = bp_core_get_incremented_cache( $folder_document_sql, 'bp_document' );

		if ( false === $cached ) {
			$document_ids = $wpdb->get_col( $folder_document_sql ); // db call ok; no-cache ok;
			bp_core_set_incremented_cache( $folder_document_sql, 'bp_document', $document_ids );
		} else {
			$document_ids = $cached;
		}

		return (array) $document_ids;
	}