diff --git a/framework/db/BaseActiveRecord.php b/framework/db/BaseActiveRecord.php
index 1bdb7ac528..bbf7da3576 100644
--- a/framework/db/BaseActiveRecord.php
+++ b/framework/db/BaseActiveRecord.php
@@ -1021,6 +1021,9 @@ abstract class BaseActiveRecord extends Model implements ActiveRecordInterface
      * @param boolean $asArray whether to return the primary key value as an array. If true,
      * the return value will be an array with column name as key and column value as value.
      * If this is false (default), a scalar value will be returned for non-composite primary key.
+     * @property mixed The old primary key value. An array (column name => column value) is
+     * returned if the primary key is composite. A string is returned otherwise (null will be
+     * returned if the key value is null).
      * @return mixed the old primary key value. An array (column name => column value) is returned if the primary key
      * is composite or `$asArray` is true. A string is returned otherwise (null will be returned if
      * the key value is null).