(Quick Reference)

7.5.2.5 Composite Primary Keys - Reference Documentation

Authors: Graeme Rocher, Peter Ledbrook, Marc Palmer, Jeff Brown, Luke Daley, Burt Beckwith, Lari Hotari

Version: 3.1.4

7.5.2.5 Composite Primary Keys

GORM supports the concept of composite identifiers (identifiers composed from 2 or more properties). It is not an approach we recommend, but is available to you if you need it:

import org.apache.commons.lang.builder.HashCodeBuilder

class Person implements Serializable {

String firstName String lastName

boolean equals(other) { if (!(other instanceof Person)) { return false }

other.firstName == firstName && other.lastName == lastName }

int hashCode() { def builder = new HashCodeBuilder() builder.append firstName builder.append lastName builder.toHashCode() }

static mapping = { id composite: ['firstName', 'lastName'] } }

The above will create a composite id of the firstName and lastName properties of the Person class. To retrieve an instance by id you use a prototype of the object itself:

def p = Person.get(new Person(firstName: "Fred", lastName: "Flintstone"))
println p.firstName

Domain classes mapped with composite primary keys must implement the Serializable interface and override the equals and hashCode methods, using the properties in the composite key for the calculations. The example above uses a HashCodeBuilder for convenience but it's fine to implement it yourself.

Another important consideration when using composite primary keys is associations. If for example you have a many-to-one association where the foreign keys are stored in the associated table then 2 columns will be present in the associated table.

For example consider the following domain class:

class Address {
    Person person
}

In this case the address table will have an additional two columns called person_first_name and person_last_name. If you wish the change the mapping of these columns then you can do so using the following technique:

class Address {
    Person person
    static mapping = {
        columns {
		    person {
                column name: "FirstName"
                column name: "LastName"
			}
        }
    }
}