Grails GORM: How do I create a composite primary key and use it for a table relationship?

Question

I have two tables, one of which (legacy table: A) has two fields that should serve as a composite foreign key and the other one (new table: B) should use a composite primary key

Answer 1

The B class has to be declared Serializable and implements the equals and hashCode methods

import org.apache.commons.lang.builder.HashCodeBuilder
class B implements Serializable{

    static auditable = true;

    String name;
    String className;
    String eventName;
    boolean equals(other) {
    if (!(other instanceof B)) {
        return false
    }

    other.className == className && other.eventName == eventName
    }

    int hashCode() {
    def builder = new HashCodeBuilder()
    builder.append className
    builder.append eventName
    builder.toHashCode()
    }

    static mapping = {
        //supposed to make a composite PK
        id composite:["className", "eventName"] 
    }
}

and the A class just must to have an attribute of the B class and the GORM make the composite foreign key

class A {

    //composite foreign key to link B class
    B b; //instance of B to be related

    static mapping = {
        table 'a_table';            
        id column: 'id';
    }
}

it create two tables in the database

+--------------------------------------------------+
|                       A_TABLE                    |  
+--------+-----------+--------------+--------------+
|   id   |  version  |  b_className | b_eventName  |
+--------+-----------+--------------+--------------+
--where the Primary key is "id" and the foreign key are "b_className and b_eventName" 
+--------------------------------------------------+
|                       B                          |  
+--------+-----------+--------------+--------------+
|  name  |  version  |  className   | eventName    |
+--------+-----------+--------------+--------------+
--where the Primary key are "className and eventName" 

if you what to change the name of the columns to other just add the clausule in the mapping statement

class A {

    //composite foreign key to link B class
    B b; //instance of B to be related

    static mapping = {
        table 'a_table';            
        id column: 'id';
        columns {
              b {
                 column name: "className"
                 column name: "eventName"
              }
        }
    }
}