what if we do not specify mappedBy attribute in OneToMany annotation?

Question

I am learning JPA with hibernate these days. I am not able to understand why hibernate gives error for a Bidirectional OnetoMany relationship if mappedBy attribute is not specified. Following is the code on which I am getting error:

@Entity
public class Item {
    @Id
    private long id;
    private String name;
    private String description;
   @OneToMany()
    Set<Bid> bids = new HashSet<Bid>();

Bid is the child entity of ITEM

@Entity(name="BIDS")
public class Bid {
    @Id
    @Column(name="BID_ID")
    private Long id;    
    private Double amount;  
    @ManyToOne
    @JoinColumn(name="ITEM_ID")
    Item item;

In the main class I am saving both parent and child entities:

Transaction transaction = session.beginTransaction();
        Item item = new Item();
        item.setId(111);
        item.setDescription("ITEM Description");
        item.setName("Name1");
        Bid bid = new Bid();
        bid.setId(21l);
        bid.setAmount(1.1);
        bid.setItem(item);
        Set<Bid> bids = new HashSet<Bid>();
        bids.add(bid);
        item.setBids(bids);
        session.save(item);
        session.save(bid);
        transaction.commit();

But hibernate tries to execute following queries and throws excetion that ITEM_BIDS table do not exist.

Hibernate: insert into Item (description, name, id) values (?, ?, ?)
Hibernate: insert into BIDS (amount, ITEM_ID, BID_ID) values (?, ?, ?)
Hibernate: insert into Item_BIDS (Item_id, bids_BID_ID) values (?, ?)

Please tell me why hibernate is generating extra query and how mappedBy element will solve this.

JB Nizet JB Nizet · Accepted Answer · 2014-07-05T08:21:47

Because if you don't specify mappedBy, you're not saying that the OneToMany between Item and Bid and the ManyToOne between Bid and Item are actually the two sides of a unique bidirectional association.

So Hibernate considers that they are two, different, unidirectional associations. And since the default mapping for a OneToMany association is to use a join table, that's what Hibernate uses.

what if we do not specify mappedBy attribute in OneToMany annotation?

1 Answers