JPA/Hibernate mapping table for One-to-many relationships

Question

I'm using JPA annotations only and hibernate 4.0.1 as the JPA implementation.

I have anAssessmentclass that has two sets of Comment:

@Entity
@Table(name = "ASSESSMENT")
public class Assessment{

    @OneToMany(fetch = FetchType.EAGER)
    private Set<Comment> auditComments = new HashSet<Comment>();

    @OneToMany(fetch = FetchType.EAGER)
    private Set<Comment> comments = new HashSet<Comment>();
}

(I'm skipping getters, setters, cascade annotations and other irrelevant lines of code)

Comment:

@Entity
@Table(name = "COMMENT")
public class Comment{

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Column(columnDefinition = "text")
    private String text;
}

Now, because a Comment doesn't have a reference back to Assessment (this is on purpose), I cannot uset the mappedBy property and Hibernate needs to create a mapping table between Comment and Assessment. By default, it is named ASSESSMENT_COMMENT.

If I had only one set of Comment in Assessment, that would work perfectly fine. The ASSESSMENT_COMMENT would have only two columns:

[ASSESSMENT_ID, COMMENT_ID]

which will perfectly represent the one-to-many relationship.

Now here is the problem:

Because I don't have one set of Comment but two, Hibernate tries to map the two sets using only one ASSESSMENT_COMMENT table. This table has 3 columns:

[ASSESSMENT_ID, COMMENT_ID, AUDITCOMMENT_ID]

On top of that, the 3 columns are set as non nullable.

That certainly cannot work. For instance, if I have only one item in auditComents and none in comments, hibernate tries to insert one row with a null in COMMENT_ID which creates a SQL exception.

Questions:

To me it looks like a bug. There should be two mapping tables, not one.

So, 1) is it a known bug in Hibernate? 2) is there a way around that? Can I force Hibernate to create two mapping tables, one for each of the set. Bear in mind that I cannot change the Comment class to have a reference to Assessment (business logic requirement)

JB Nizet · Accepted Answer · 2012-12-14 10:38:17Z

8

First of all, you could use two columns in the comment table to point back to their assessment:

@OneToMany(fetch = FetchType.EAGER)
@JoinColumn(name = "audit_commented_assessment_id")
private Set<Comment> auditComments = new HashSet<Comment>();

@OneToMany(fetch = FetchType.EAGER)
@JoinColumn(name = "commented_assessment_id")
private Set<Comment> comments = new HashSet<Comment>();

If you want two join tables, the just say so:

@OneToMany(fetch = FetchType.EAGER)
@JoinTable(name = "assessment_audit_comment",
           joinColumns = @JoinColumn(name = "assessment_id"),
           inverseJoinColumns = @JoinColumn(name = "comment_id"))
private Set<Comment> auditComments = new HashSet<Comment>();

@OneToMany(fetch = FetchType.EAGER)
@JoinTable(name = "assessment_comment",
           joinColumns = @JoinColumn(name = "assessment_id"),
           inverseJoinColumns = @JoinColumn(name = "comment_id"))
private Set<Comment> comments = new HashSet<Comment>();

That is of course explained in the documentation.

answered Dec 14, 2012 at 10:38

JB Nizet

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5 Comments

phoenix7360 Over a year ago

Thanks it fixed it. I never had two collection of the same type so I never had to use the @JoinTable annotation.

nize Over a year ago

How would the Comment class look if you would like to have a bidirectional relation? E.g. in the Comment POJO I would like to have a reference to an Assessment? You can not use two @jointable (one against each mapping table of the parent)? How would you do it?

JB Nizet Over a year ago

@nize: If the associations were bidirectional, you would have to make Comment the owner side of the associations. In Comment, you would have a field commentedAssessment and a field auditedAssessment. They would be mapped with @ManyToOne, with a join table as above, but with joinColumns and inverseJoinColumns switched. In Assessment, you would simply have @OneToMany(mappedBy = "auditedAssessment") private Set<Comment> auditComments; and @OneToMany(mappedBy = "commentedAssessment") Set<Comment> comments;

nize Over a year ago

Ok, but assume the case is that a comment must either be a auditComment OR a comment, not both. That is why I would like to have the Comment to only have a reference "Assessment parent", not two references "commentedAssessment" and "auditedAssessment". Would that be possible to achieve or do I need to have two references of which one always will be NULL? Maybe then I need to extend Comment with two classes, AuditComment and NormalComment?

nize Over a year ago

I tried that path, but I experienced it to be a mess since the mappedBy reference to the superclass didn't work as I expected. I decided to try another alternative: In the class Assessment I have the List<Comment> comments. In the Comment class I use a boolean isAuditComment. Then I can provide methods in the Assessment class like List<Comment> getAuditingComments() and List<Comment> getOtherComments(). Not as elegant, but less complex from a JPA perspective.

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