package test;

import java.util.HashMap;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.MappedSuperclass;
import javax.persistence.NoResultException;
import javax.persistence.Persistence;
import javax.persistence.Query;
import javax.persistence.Table;

public class Main {

    protected static EntityManager em;
    protected static EntityManager em2;

    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("test", new HashMap<Object, Object>());
        em = emf.createEntityManager();
        em2 = emf.createEntityManager();

        User alice = new User("alice");
        Foo foo = new Foo();
        Bar bar = new Bar();
        foo.bar = bar;

        create(alice);
        create(foo);
        create(bar);
    }

    public static User getUser() {
        Query q = em2.createQuery("select x from User x where x.name='alice'");
        try {
            return (User) q.getSingleResult();
        } catch (NoResultException e) {
            return null;
        }
    }

    public static void create(BaseEntity entity) {
        entity.createdBy = getUser();
        EntityTransaction tx = em.getTransaction();
        tx.begin();
        em.persist(entity);
        tx.commit();
    }
}

@MappedSuperclass
abstract class BaseEntity {
    @Id
    @GeneratedValue
    Long id;

    @ManyToOne
    User createdBy;
}

@Entity
@Table(name = "user2")
class User extends BaseEntity {

    String name;

    public User() {
    }

    public User(String name) {
        this.name = name;
    }
}

@Entity
class Foo extends BaseEntity {
    @ManyToOne(cascade = CascadeType.PERSIST)
    Bar bar;
}

@Entity
class Bar extends BaseEntity {

}

<persistence ...>
    <persistence-unit name="test" transaction-type="RESOURCE_LOCAL">
        <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>

        <class>test.BaseEntity</class>
        <class>test.User</class>
        <class>test.Foo</class>
        <class>test.Bar</class>

        <properties>
            <property name="javax.persistence.jdbc.driver" value="org.postgresql.Driver" />
            <property name="javax.persistence.jdbc.url" value="jdbc:postgresql://localhost/testdb" />
            <property name="javax.persistence.jdbc.user" value="postgres" />
            <property name="javax.persistence.jdbc.password" value="postgres" />
            <property name="eclipselink.weaving" value="false" />
            <property name="eclipselink.ddl-generation" value="create-tables" />
            <property name="eclipselink.target-database" value="PostgreSQL" />
            <property name="eclipselink.logging.level" value="ALL" />
        </properties>
    </persistence-unit>

</persistence>