JPA and Hibernate : one to one mapping causes three select queries

Question

JPA 2.0 Hibernate 4.3.5

Hi,

Following is my OneToOne mapping (with example code assuming 1 customer can only have 1 order)

class Customer {
    private Order order;
    @OneToOne(mappedBy="customer", fetch=FetchType.LAZY)
    public Order getOrder() { return order; }
    public void setOrder(Order order) { this.order = order ; }
}

class Order {
    private Customer customer;
    @OneToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="cust_id")
    public Customer getCustomer() { return customer; }
    public void setCustomer(Customer customer) { this.customer = customer; }
}

//Calling code
Order order = em.find(Order.class, 4);    // Line 1
System.out.println(order.getCustomer());  // Line 2
</code>

Above calling code is actually resulting in 3 select statements i.e

Line 1 causing

   select * from order where id = ?      #1

Line 2 causing following two statements

   select * from customer where id = ?   #2
   select * from order where cust_id = ? #3

My Question: Shouldn't it be only two queries (i.e #1 and #2) given that the LAZY fetching is enabled on both ends ?

Thanks, Rakesh

Answer 1

You have defined both relationships as LAZY.

When you load the Order the Customer attribute will not be loaded because it is LAZY. A LAZY attribute will only be loaded when it is accessed or if you define it to be eagerly loaded.

Every mapping finished with one (OneToOne or ManyToOne) will be eagerly by default, but you set it as LAZY.

Try to define your relationship as EAGER:

@OneToOne(fetch=FetchType.EAGER) // or just @OneToOne
@JoinColumn(name="cust_id")
public Customer getCustomer() { return customer; }

By the way, your customer can have only one order? O.o