Hibernate mapping with group by clause?

Question

I have @OneToMany mapping in a pojo, and I need to group the resulting list by a column. I have a mapping like the following;

@OneToMany(fetch = FetchType.EAGER)
@JoinColumn(name = "X", referencedColumnName = "X", insertable = false, updatable = false)
@NotFound(action = NotFoundAction.IGNORE)
@Fetch(FetchMode.SELECT)
public Set<MyType> getSiblings() {
    return siblings;
}

I need to add GROUP BY clause to this mapping, is this possible?

Answer 1

I think what you are looking for is support for mapping into a multimap, which is a map with one key and the value is a list of values all associated to the same key.

This is not supported by Hibernate, this is a thread in their forum with an attempt to implement it.

An alternative is to still map to a Set at the property level, and then provide a getter that does the grouping on the fly. This would be transparent for consumers and only become a problem for very large datasets:

@OneToMany(fetch = FetchType.EAGER)
@JoinColumn(name = "X", referencedColumnName = "X", insertable = false, updatable = false)
@NotFound(action = NotFoundAction.IGNORE)
@Fetch(FetchMode.SELECT)
private Set<MyType> siblings;

public Map<String,List<MyType>> getSiblings() {
   ... do the grouping manually in Java ...
}