Question

Hello everybody i wonder: when can i use mapped by to indicate whose is owing of relationship in one-to -one or one to many – or many to many relationship mapping with EJB3 (JPA)
example
i have two table A and B
table A belong to table B
so what i place mapped by for whose table?

Answer 1

when can I use mapped by to indicate whose is owing of relationship in one-to -one or one to many – or many to many relationship mapping with EJB3

A relationship can be unidirectional or bidirectional. Within a bidirectional relationship you must specify the owning side of the relationship in the other class with the mappedBy element.

• The owning side which is responsible for propagating the update of the relationship to the database. Usually this is the side with the foreign key.
• The inverse side maps to the owning side.

From the JPA 1.0 specification:

2.1.7 Entity Relationships …

Relationships may be bidirectional or
unidirectional. A bidirectional
relationship has both an owning side
and an inverse side. A unidirectional
relationship has only an owning side.
The owning side of a relationship
determines the updates to the
relationship in the database, as
described in section 3.2.3.

The following rules apply to
bidirectional relationships:

• The inverse side of a bidirectional relationship must refer to its owning
  side by use of the mappedBy element
  of the OneToOne, OneToMany, or
  ManyToMany annotation. The
  mappedBy element designates the
  property or field in the entity that
  is the owner of the relationship.
• The many side of one-to-many / many-to-one bidirectional
  relationships must be the owning side,
  hence the mappedBy element cannot be
  specified on the ManyToOne annotation.
• For one-to-one bidirectional relationships, the owning side
  corresponds to the side that contains
  the corresponding foreign key.
• For many-to-many bidirectional relationships either side may be the
  owning side.

Imagine the following model:

@Entity
public class Player {
...
    private Team team;

    @ManyToOne
    public Team getTeam() { return team; }

    ...
} 

And

@Entity
public class Team {
    ...    
    private Set<Player> players = new HashSet<Player();

    public Team() { }

    @OneToMany(mappedBy = "team")
    public Set<Player> getPlayers() { return players; }

    ...    
}

In this example, the mappedBy attribute shows that a Player instance’s team property maps to the Team instance and the Team object’s identifier will exist as a foreign key column in the PLAYER table. The owning Player side of the relationship is responsible for storing the foreign key.

If the mappedBy is not used, the persistence provider will assume that there are two independent relationships:

Which is generally not what you want and might end up getting unexpected behavior (e.g. duplicate rows inserted with many-to-many).

Related questions

• In a bidirectional JPA OneToMany/ManyToOne association, what is meant by “the inverse side of the association”?
• JPA: which side should be the owning side in a m:n relationship?

References

• JPA 1.0 specification
  • Section 2.1.7 “Entity Relationships”