JPA OneToMany and ManyToOne with a composite key is generating a third table

Question

I'm mapping a Filter ---< FilterColumn where Filter presents cardinality one and FilterColumn N. So the mapped classes are:

@Entity
public class Filter implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    private String name;
    private String caption;
    @OneToMany(cascade = CascadeType.MERGE, targetEntity = FilterColumn.class)
    private Set<FilterColumn> columns;

    // setters and getters

}



@Entity
public class FilterColumn implements Serializable {

    private static final long serialVersionUID = 1L;

    @EmbeddedId
    private FilterColumnId id;
    private String caption;

    // getters and setters

@Embeddable
public static class FilterColumnId implements Serializable {

    private static final long serialVersionUID = 1L;

    @ManyToOne
    private Filter filter;
    @Column
    private String name;

    // getters and setters

}


}

But when I start the application with drop-create instruction the following 3 tables are created: Filter PK(name) FilterColumn PK(filter_name, name) Filter_FilterColumn PK(filter_filter_name, filterColumn_filter_name, filterColumn_name)

What I really want is just two tables like: Filter PK(name) Filter_Column PK(name, filter_name)

Why do I receive this result? Is there something wrong with my mapping? What should I change?

Thanks in advance.

Answer 1

I think you need a mappedBy on the @OneToMany . Without that, the mapper doesn't know that it can look at the filtercolumn table to find the entities associated with a Filter , so it generates the filter_filtercolumn table.

Not sure off the top of my head how you to a mappedBy with a composite key. Given that you're using an @EmbeddedId , i think it's simply mappedBy = "id" .

Can you use a @ManyToOne in a key class like that? Is that a Hibernate extension over and above the JPA spec? Wouldn't you normally need a @MapsId in there somewhere?

Answer 2

Try adding a @JoinColumn annotation on the Filter member of your composite id. The actual column would be whatever the id of the of the Filter table is (or just leave it without a name if you let hibernate generate it all).

Let me know if this works as I had a similar problem and solved it using the above so I do know it's possible. The only other thing mine has is a @ForeignKey annotation but I think hibernate will take care of that for you -- I just did mine because I wanted to stick to a naming convention.