inheritance in Grails domain model

Question

My Grails app's domain model has the following requirements:

• a user belong to zero or one organisations
• an organisation is either a charity or a company
• charities and companies have some some common fields and also some (non-nullable) fields that are unique to each organisation type

I put the common organisation fields into an abstract Organisation class which Charity and Company both extend. I can't store this hierarchy in a single table because there are non-nullable fields that are specific to each organisation type. The relevant parts of the domain model are shown below:

class User {
  String name

  static belongsTo = [organization: Organization]

  static constraints = {
    organization nullable: true
  }
}

abstract class Organization {    
    String name

    static hasMany = [users: User]

    static mapping = {
        tablePerHierarchy false
    }
}

class Charity extends Organization {
  // charity-specific fields go here
} 

class Company extends Organization {
  // company-specific fields go here
}

When I look at the MySQL schema generated from this model, the inheritance relationship between organisation-company and organisation-charity seems to have been completely ignored. Although there is an organisation table with a name column, it has no primary-foreign key relationship with either company or charity

Answer 1

Solved!

Add the class below to src/java (this class cannot be written in Groovy)

package org.example;

import org.codehaus.groovy.grails.orm.hibernate.cfg.GrailsAnnotationConfiguration;
import org.hibernate.MappingException;
import org.hibernate.mapping.JoinedSubclass;
import org.hibernate.mapping.PersistentClass;
import org.hibernate.mapping.RootClass;

import java.util.Iterator;

public class TablePerSubclassConfiguration extends GrailsAnnotationConfiguration {

    private static final long serialVersionUID = 1;

    private boolean alreadyProcessed = false;

    @Override
    protected void secondPassCompile() throws MappingException {
        super.secondPassCompile();

        if (alreadyProcessed) {
            return;
        }

        for (PersistentClass persistentClass : classes.values()) {
            if (persistentClass instanceof RootClass) {
                RootClass rootClass = (RootClass) persistentClass;

                if (rootClass.hasSubclasses()) {
                    Iterator subclasses = rootClass.getSubclassIterator();

                    while (subclasses.hasNext()) {

                        Object subclass = subclasses.next();

                        // This test ensures that foreign keys will only be created for subclasses that are
                        // mapped using "table per subclass"
                        if (subclass instanceof JoinedSubclass) {
                            JoinedSubclass joinedSubclass = (JoinedSubclass) subclass;
                            joinedSubclass.createForeignKey();
                        }
                    }
                }
            }
        }

        alreadyProcessed = true;
    }
}

Then in DataSource.groovy set this as the configuration class

dataSource {
    configClass = 'org.example.TablePerSubclassConfiguration'
    pooled = true
    driverClassName = "org.h2.Driver"
    username = "sa"
    password = ""
    dbCreate = "update"
    url = "jdbc:h2:mem:testDb;MVCC=TRUE;LOCK_TIMEOUT=10000"
}

I've submitted a pull request to Grails that fixes this. The fix was be included in Grails 2.3.9.