How to store recursive object in database?

Question

How can I store an object that has references to other objects of the same class?

I have tried a lot of ways to doing it with annotations and XML but all of them have failed.

In the best cases, I get the following error:

WARNING: Insert of object "pmo.domain.entities.Unit@1129829c" using statement "INSERT INTO UNIT ("NAME",PARENT_UNIT_ID,ID) VALUES (?,?,?)" failed : Parameter "#2" is not set; SQL statement: INSERT INTO UNIT ("NAME",PARENT_UNIT_ID,ID) VALUES (?,?,?) [90012-196]

ExampleTest.java:

package pmo.domain.entities;

public class ExampleTest {
    public static void Main(String[] args) {
        Unit unit1 = new Unit(0, "Unit 1");
        Unit unit2 = new Unit(1, "Unit 2", unit1);
        Unit unit3 = new Unit(2, "Unit 3", unit2);

        PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory("Test");
        PersistanceManager pm = pmf.getPersistenceManager();

        Transaction transaction = pm.currentTransaction();

        try {
            transaction.begin();
            pm.makePersistent(unit1);
            pm.makePersistent(unit2);
            pm.makePersistent(unit3);
            transaction.commit();
        } finally {
            if (transaction.isActive()) {
                transaction.rollback();
            }

            pm.close();
        }
    }
}

Unit.java:

package pmo.domain.entities;

public class Unit {
    private long id;
    private String name;
    private Unit parentUnit;

    public Unit(long id, String name, Unit parentUnit) {
        this.id = id;
        this.name = name;
        this.parentUnit = parentUnit;
    }

    public Unit(long id, String name) {
        this(id, name, null);
    }
}

package.jdo:

<?xml version="1.0" encoding="UTF-8" ?>
<jdo xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xmlns="http://xmlns.jcp.org/xml/ns/jdo/jdo"
     xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/jdo/jdo http://xmlns.jcp.org/xml/ns/jdo/jdo_3_1.xsd" version="3.1">
    <package name="pmo.domain.entities">
        <class name="Unit" table="unit">
            <field name="id" primary-key="true">
                <column name="id"/>
            </field>
            <field name="parentUnit">
                <column name="parent_unit_id" target-field="id"/>
                <foreign-key name="unit_id_fk" />
            </field>
        </class>
    </package>
</jdo>

You can't store a record with a key to another record, that does not exist, whether or not it is of the same class. In you case , your first save will fail, because the parent is null — jr593, Jan 05 '18 at 08:51
@jr593 In other testings that I'm doing without parentUnit filled is working nicely. It is only in the case shown that it fails. I'm looking for ways to implement this relation in another way. — Facon, Jan 05 '18 at 09:10

score 0 · Accepted Answer · answered Jan 05 '18 at 10:31

I have solved the issue by inverting the relation. From child to parent. So now, it is parent to child using a collection.

ExampleTest.java:

package pmo.domain.entities;

public class ExampleTest {
    public static void Main(String[] args) {
        Unit unit1 = new Unit(0, "Unit 1");
        Unit unit2 = new Unit(1, "Unit 2");
        Unit unit3 = new Unit(2, "Unit 3");

        unit2.addUnit(unit3);
        unit1.addUnit(unit2);

        PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory("Test");
        PersistanceManager pm = pmf.getPersistenceManager();

        Transaction transaction = pm.currentTransaction();

        try {
            transaction.begin();
            pm.makePersistent(unit1);
            transaction.commit();
        } finally {
            if (transaction.isActive()) {
                transaction.rollback();
            }

            pm.close();
        }
    }
}

Unit.java:

package pmo.domain.entities;

public class Unit {
    private long id;
    private String name;
    private Collection<Unit> childUnits;

    public Unit(long id, String name, Collection<Unit> childUnits) {
        this.id = id;
        this.name = name;
        this.childUnits = childUnits;
    }

    public Unit(long id, String name) {
        this(id, name, new ArrayList<>());
    }

    public void addUnit(Unit unit) {
        this.childUnits.add(unit);
    }

    public void removeUnit(long id) {
        childUnits.removeIf(unit -> unit.id == id);
    }
}

package.jdo:

<?xml version="1.0" encoding="UTF-8" ?>
<jdo xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xmlns="http://xmlns.jcp.org/xml/ns/jdo/jdo"
     xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/jdo/jdo http://xmlns.jcp.org/xml/ns/jdo/jdo_3_1.xsd" version="3.1">
    <package name="es.indra.pmo.pmomanagement.domain.entities">
        <class name="Employee" table="employee">
            <field name="id" primary-key="true"/>
        </class>
        <class name="Unit" table="unit">
            <field name="id" primary-key="true">
                <column name="id"/>
            </field>
            <field name="childUnits">
                <order column="parent_unit_id"/>
            </field>
        </class>
    </package>
</jdo>

How to store recursive object in database?

1 Answers1