//Parent.java
    @Entity
    @NoSql(dataFormat = DataFormatType.MAPPED)
    public class Parent {
        @Id
        @GeneratedValue
        @Field(name = "_id")
        private String id;
        private String parentName;

        @OneToOne(cascade = { CascadeType.ALL }, fetch = FetchType.LAZY)
        private Child  child;

        public String getId() {
            return id;
        }

        public void setId(String id) {
            this.id = id;
    }

        public String getParentName() {
           return parentName;
       }

        public void setParentName(String parentName) {
           this.parentName = parentName;
        }

    public Child getChild() {
        return child;
    }

    public void setChild(Child child) {
        this.child = child;
    }

    }



    //Child.java
    @Entity
    @NoSql(dataFormat = DataFormatType.MAPPED)
    public class Child {
    @Id
    @GeneratedValue
    @Field(name = "_id")
    private String id;

    private String childName;

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getChildName() {
        return childName;
    }

    public void setChildName(String childName) {
        this.childName = childName;
    }

   }


    //OneToOneTest.java - this has the main()
    public class OneToOneTest {

    static EntityManagerFactory    factory;
    protected static EntityManager em;

    public OneToOneTest() {
        Map<String, Object> properties = new HashMap<String, Object>();
        properties.put(PersistenceUnitProperties.CLASSLOADER,          OneToOneTest.class.getClassLoader());

        factory = new PersistenceProvider().createEntityManagerFactory("mongo", properties);
        em = factory.createEntityManager();
    }

    public Parent createParent(Parent parent) {
        em.getTransaction().begin();
        em.persist(parent);
        em.getTransaction().commit();
        return parent;
    }

    public Child createChild(Child child) {
        em.getTransaction().begin();
        em.persist(child);
        em.getTransaction().commit();
        return child;
    }

    public Parent readParent(String id) {
        Parent p = em.find(Parent.class, id);
        return p;
    }

    public Child readChild(String id) {
        Child c = em.find(Child.class, id);
        return c;
    }

    public Parent updateEntity(final Parent entity) {
        em.getTransaction().begin();
        Parent merged = em.merge(entity);
        em.flush();
        em.getTransaction().commit();
        return merged;
    }

    public static void main(String[] args) throws Exception {
        OneToOneTest daoService = new OneToOneTest();
        Map<String, Object> properties = new HashMap<String, Object>();
        properties.put(PersistenceUnitProperties.CLASSLOADER, OneToOneTest.class.getClassLoader());
        daoService.factory = new PersistenceProvider().createEntityManagerFactory("mongo", properties);

        EntityManager em = daoService.factory.createEntityManager();
        // First clear old database.
        em.getTransaction().begin();
        DB db = ((MongoConnection) em.unwrap(javax.resource.cci.Connection.class)).getDB();
        db.dropDatabase();
        em.getTransaction().commit();

        Parent parent = new Parent();
        parent.setParentName("pname");

        // Create
        System.out.println("***************  Persisting *****************");

        Child child = new Child();
        child.setChildName("cname");

        Parent p = daoService.createParent(parent);

        p.setChild(child);

        daoService.updateEntity(p);

        Parent parent2 = daoService.readParent(p.getId());

        System.out.println("Child : " + parent2.getChild());

        Child c = daoService.readChild(parent2.getChild().getId());
        System.out.println("Child id : " + c.getId()); //Child 'id' is printed correctly here.

        daoService.factory.close();
    }