grails-documentation-simpledb/src/docs/guide/mapping/identityGeneration.gdoc

The plugin works only with String identifiers. Assignment and generation of ids is done automatically, however the String
id field must be currently explicitly declared in the domain class:

{code}
class Person {
    String id
    String firstName
    String lastName

    static mapWith = "simpledb"
}
{code}

By default, generated ids are generated with @java.lang.UUID@. It is also possible to use hilo numeric value generator
(please note that the id field must still be declared as String):
{code}
class Person {
    String id
    String firstName
    String lastName

    static mapWith = "simpledb"

    static mapping = {
        id_generator type: 'hilo', maxLo: 500
    }
}
{code}