使用继承时，Hibernate ManyToMany不起作用

Question

I just refactor a Project to use Hibernate (4.2.4.Final) with Inheritance. But I got trouble with ManyToMany annotation.

I have a base File Class like this:

@Entity
@Table(name = "file")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "descriminator", length = 25)
public abstract class File {

    @Id
    @Column(name = "id", unique = true, nullable = false, length = 256)
    private String id;
}

and a special Inheritance class like this:

@Entity
@DiscriminatorValue("ISSUE_HISTORY_ATTACHMENT")
@Data
public class IssueHistoryAttachment extends File {

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinTable(name = "issue_history_attachment", joinColumns = {
            @JoinColumn(name = "attachment_id", nullable = false, unique = true) }, inverseJoinColumns = {
            @JoinColumn(name = "issue_history_id", nullable = false)})
    private IssueHistory history;

}

This IssueHistoryAttachment Class is also referenced in my IssueHistory Class.

@Entity
@Table(name = "issue_history")
@TableGenerator(name="tg", table="hibernate_sequences",pkColumnName="sequence_name", valueColumnName="sequence_next_hi_value", allocationSize=1)
public class IssueHistory implements java.io.Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.TABLE, generator = "tg")
    @Column(name = "id", unique = true, nullable = false)
    private int id;

// some other fields

    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "issue_history_attachment", joinColumns = {
            @JoinColumn(name = "issue_history_id", nullable = false)
        }, inverseJoinColumns = {
            @JoinColumn(name = "attachment_id", nullable = false, unique = true)
    })
    private Set<IssueHistoryAttachment> attachments = new HashSet<IssueHistoryAttachment>();

}

When i now store a IssueHistory Instance with two Attachments, all this fields are correctly saved in my database.

I got 2 new entries in the file table, one new entry in the *issue_history* table and two correct entries in the relation table *issue_history_attachment*.

So at this points all thinks are looking fine. But when I try to read the Values Attachment Set in the IssueHistory Instance only contains one element instead of two like stored in the database.

Any suggestions how to solve this?

Answer 1

I just found the source of the Problem.

It was a missing/wrong equals method. :-)

Answer 2

I can't comment yes so I have to make an answer.

I see one problem in your code (or maybe I don't understand it):

In IssueHistory you are using @ManyToMany to IssueHistoryAttachment but in IssueHistoryAttachment you are using @ManyToOne .

In my opinion it is the reason of your problem.