Question

I am trying to generate a unidirectional one-to-many mapping between two JPA entities. In my ChatRoom class, I have a Vector<User> type object that represents Users in a ChatRoom. I am trying to create a one-to-many mapping, based on userId (in the User class). Here is my sample code:

@Entity
public class ChatRoom {
    @Id
    @GeneratedValue
    private int chatRoomID;

    @OneToMany(mappedBy="userID")
    private Vector<User> users;

    // A no-argument constructor is required for EclipseLink to instantiate the persistence object properly.
    public ChatRoom() {}

    // Getter/Setter section
}

Here is the User class:

@Entity
public class User {
    @Id
    @GeneratedValue
    private int userID;

    private String username;

    // Getter/Setter section
}

When I try to generate the tables from these entities in Eclipse, I get the following error:

Internal Exception: javax.persistence.PersistenceException: Exception [EclipseLink-28018] (Eclipse Persistence Services - 2.0.2.v20100323-r6872): org.eclipse.persistence.exceptions.EntityManagerSetupException
Exception Description: Predeployment of PersistenceUnit [ChatJPA] failed.
Internal Exception: Exception [EclipseLink-7244] (Eclipse Persistence Services - 2.0.2.v20100323-r6872): org.eclipse.persistence.exceptions.ValidationException
Exception Description: An incompatible mapping has been encountered between [class iosoa.entity.ChatRoom] and [class iosoa.entity.User]. This usually occurs when the cardinality of a mapping does not correspond with the cardinality of its backpointer.
    at org.eclipse.persistence.exceptions.PersistenceUnitLoadingException.exceptionSearchingForPersistenceResources(PersistenceUnitLoadingException.java:126)

Do you have any ideas on how to resolve this problem? Thanks for your help.

Answer 1

A few things,

Your mappedBy is wrong, a mappedBy for a OneToMany must be a ManyToOne. I would recommend you add the ManyToOne back, or use a @JoinTable, if you do not wish to, then you could use an @JoinColumn and no mappedBy.

Vector is not valid in JPA, you must use List, Set or Map. You can still use a Vector as your implementation class. If you really want to use Vector, then EclipseLink will support this, but you must make your mapping EAGER.