grails-documentation-mongodb/src/docs/guide/3.1 Identity Generation.gdoc

By default in GORM entities are supplied with an integer-based identifier. So for example the following entity:

{code}
class Person {}
{code}

Has a property called @id@ of type @java.lang.Long@. In this case GORM for Mongo will generate a sequence based identifier using the technique [described in the Mongo documentation|http://docs.mongodb.org/manual/tutorial/isolate-sequence-of-operations/] on Atomic operations.

However, sequence based integer identifiers are not ideal for environments that require [sharding|http://docs.mongodb.org/manual/sharding/] (one of the nicer features of Mongo). Hence it is generally advised to use either String based ids:

{code}
class Person {
    String id
}
{code}

Or a native BSON [ObjectId|http://api.mongodb.org/java/current/org/bson/types/ObjectId.html]:

{code}
import org.bson.types.ObjectId

class Person {
    ObjectId id
}
{code}

BSON @ObjectId@ instances are generated in a similar fashion to @UUIDs@.

h4. Assigned Identifiers

Note that if you manually assign an identifier, then you will need to use the @insert@ method instead of the @save@ method, otherwise GORM can't work out whether you are trying to achieve an insert or an update. Example:

{code}
class Person {
    String id
}
...
def p = new Person(id:"Fred")
// to insert
p.insert()
// to update
p.save()
{code}