MongoDB Documentation, Release 3.0.4 findAndModify Returns Null Value for Upserts that Perform Inserts In version 2.2, for upsert that perform inserts with the new option set to false , findAndModify commands will now return the following output: In the mongo shell, upsert findAndModify operations that perform inserts (with new set to false .)only output a null value. In version 2.0 these operations would return an empty document, e.g. { } . See: SERVER-6226 831 for more information. • In sharded environments, to ensure that your cluster remains available during the upgrade process you must use the upgrade procedure for sharded clusters (page 901). mongodump 2.2 Output Incompatible with Pre-2.2 mongorestore If you use the mongodump tool from the 2.2 distribution to create a dump of a database, you must use a 2.2 (or later) version of mongorestore to restore that dump. See: SERVER-6961 832 for more information. ObjectId().toString() Returns String Literal ObjectId("...") In version 2.2, the toString() method returns the string representation of the ObjectId() (page 176) object and has the format ObjectId("...") . Consider the following example that calls the toString() method on the ObjectId("507c7f79bcf86cd7994f6c0e") object: ObjectId("507c7f79bcf86cd7994f6c0e").toString() The method now returns the string ObjectId("507c7f79bcf86cd7994f6c0e") . { 'ok': 1.0, 'value': null } ObjectId().valueOf() Returns hexadecimal string In version 2.2, the valueOf() method returns the value of the ObjectId() (page 176) object as a lowercase hexadecimal string. Consider the following example that calls the valueOf() method on the ObjectId("507c7f79bcf86cd7994f6c0e") object: ObjectId("507c7f79bcf86cd7994f6c0e").valueOf() The method now returns the hexadecimal string 507c7f79bcf86cd7994f6c0e . Previously, in version 2.0, the method would return the object ObjectId("507c7f79bcf86cd7994f6c0e") . If compatibility between versions 2.0 and 2.2 is required, use ObjectId().str (page 176) attribute, which holds the hexadecimal string value in both versions. If compatibility between versions 2.0 and 2.2 is required, use ObjectId().str (page 176), which holds the hexadecimal string value in both versions. Previously, in version 2.0, the method would return the hexadecimal string 507c7f79bcf86cd7994f6c0e . 903 12.2. Previous Stable Releases $^{831}$https://jira.mongodb.org/browse/SERVER-6226 $^{832}$https://jira.mongodb.org/browse/SERVER-6961