what us mean mappedBy="item" ?

[Samanthi perera]

this is from manning action ejb 3 book.page no 245

@Entity
public class Item {
@Id
protected Long itemId;
protected String title;
protected String description;
protected Date postdate;
...
@OneToMany(mappedBy="item")
protected Set<Bid> bids;
...
}
@Entity
public class Bid {
@Id
protected Long bidId;
protected Double amount;
protected Date timestamp;
...
@ManyToOne
protected Item item;
...
}

i am refering to this line @OneToMany(mappedBy="item")
what us mean mappedBy="item" ?

[Uppala Ramana]

i am refering to this line @OneToMany(mappedBy="item")
what us mean mappedBy="item" ?

Samanthi,

The mappedBy element identifies the inverse side of a relationship.

The inverse side of the bidirectional relationship (OneToOne,OneToMany,ManyToMany) refer to its owining side by using the mappedBy element.

In your example, mappedBy="item" is the inverse side of the relation that is Item-> Bids and Bid->Item is the Owning side.

To fetch Bid from object from the Item class, JPA uses the attribute or property specified in the mappedBy element.


Thanks,
Ramana.

[Samanthi perera]

still i don't understood what you said.
what is you mean by owning?

[Jose Ayerdis]

well mappedBy anotation is very simple let me give you this sceneraio:

I have a Vehicle and Driver POJO's both represent entities on the Database both of them are mapped like this:

@Entity
@Table(name = "vehicle", catalog = "mycatalog")
public class Vehicle implements java.io.Serializable {

    private String id;
   @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "id_transportista", nullable = false)
    private Driver transportista;
    private String description;
    private String color;
    private Float weight;
....
}

@Entity
@Table(name="driver",catalog="mycatalog")
public class Transportista  implements java.io.Serializable {


     private Integer idTransportista;
     private String nombre;
     private String direccion;
@OneToMany(cascade=CascadeType.ALL, fetch=FetchType.LAZY, mappedBy="transportista")
     private Set<Vehicle> vehiculos = new HashSet<Vehicle>(0);
.....

As you can see the mappedBy is the refering to a "transportista" object in Vehicle class which said something like this....

"This Set of Vehicle is OneToMany relationship with the class (Vehicle) and do a mapped Inverse mapping with an object named transportista"

So it goes to transportista and gets this

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "id_transportista", nullable = false)
private Driver transportista;


you'll see now hibernate does not have to guess about the reference and that is inverse mapping..............

[Christian Dillinger]

If you have a bi-directional relation between two classes Hibernate "thinks" it has to maintain both sides of relation. So each operation on the relation would lead to two SQL-statements. If you declare one side as "inverse" (by using mappedBy), Hibernate ignores changes made on that side.

I recommend reading "Java Persistence with Hibernate" which is a really great book about Hibernate. All the questions on Hibernate you posted are answered in that book.