Identifiers

All about entity identifiers, from primary keys, and generators to composite keys and field types.

Identifier properties are denoted via fieldtype="id".

We highly recommend disabling updates on identifier properties via update="false":

property name      = "id"
         fieldtype = "id"
         ormtype   = "string"
         generator = "assigned"
         update    = "false";

Common Generator Types

Assigned Generator

The Assigned generator is the default identifier generator type, and simply allows the application (your boxlang code) to assign identifier values prior to insertion. You can think of this as generator=none.

lets the application assign an identifier to the object before save() is called. This is the default strategy if no element is specified. - Hibernate 3.3 mapping reference docs

property name      = "id"
         fieldtype = "id"
         generator = "assigned"
         update    = "false";

Assigned generators will commonly use a preInsert() event listener to assign the identifer value:

function preInsert( entity ){
    setId( createUUID() );
}

Select Generator

A select generator will attempt to select the next identifier value from the specified selectKey column:

retrieves a primary key, assigned by a database trigger, by selecting the row by some unique key and retrieving the primary key value. - Hibernate 3.3 mapping reference docs

property name      = "userID"
         fieldtype = "id"
         generator = "select"
         selectKey = "ssn"
         update    = "false";

UUID Generator

The UUID generator uses Hibernate's uuid generator under the hood:

uses a 128-bit UUID algorithm to generate identifiers of type string that are unique within a network (the IP address is used). The UUID is encoded as a string of 32 hexadecimal digits in length. - Hibernate 3.3 mapping reference docs

property name      = "userID"
         fieldtype = "id"
         generator = "uuid"
         update    = "false";

Increment Generator

The Increment generator uses a simple incrementing value to create identifiers. Do not use in concurrent applications, as the values may no longer be unique.

Generates identifiers of type long, short or int that are unique only when no other process is inserting data into the same table. Do not use in a cluster. - Hibernate 3.3 mapping reference docs

property name      = "userID"
         fieldtype = "id"
         generator = "increment"
         update    = "false";

Other Valid Generator Types

There are several lesser-known identifier generator types, including:

• foreign - use a specific property from a foreign entity

• seqhilo - use an optimized hi/lo algorithm to generate values from a specified sequence

• sequence - Alias for seqhilo

• identity - use an identity column from the database (auto-increment)

• native - use identity, sequence or hilo depending on the underlying database capabilities

• select - select the next value from the column denoted in selectKey

• uuid - use Hibernate's UUID generation

See the Hibernate 3.3 Mapping Reference Documentation for more information on these and other generator types.