I have a question about how to select a set of distinct IDs from a set of objects a Hibernate object object references. For this example, assume I have a Person class, and a Child class. A Person has a set of Child (as seen in Person class below). Each Child has an ID.
Code:
@Entity
public class Person {
private Set<Child> _children = new HashSet<Child>();
private String name;
@ManyToMany(targetEntity=Child.class, cascade=CascadeType.ALL)
@JoinTable(
name="CHILD_MAP",
joinColumns={@JoinColumn(name="PERSONID")},
inverseJoinColumns={@JoinColumn(name="CHILDID")}
)
public Set<Child> getChildren() {
return _children;
}
public void setChildren(Set<Child> children) {
this._children = children;
}
}
Before Hibernate, I was able to do a query something like this, to get a distinct list of the parent's name, and all children IDs:
Code:
SELECT DISTINCT
P.name,
CMAP.childid
FROM
child_map CMAP,
person P
WHERE
CMAP.personid = P.id
However, I am confused about how to do this with HQL.
I thought that I could do this in HQL:
Code:
select distinct
p.children.id,
p.name
from
Parent p
This does not seem to work, and I get an error like:
Code:
Invalid path: 'p.children.id'
How do I work with a set when I want to get distinct elements back in a join like this? I have seen examples where the set is used in the WHERE clause to filter results (e.g. WHERE exists elements(p.children)), but none of these examples seem applicable to my problem. There is probably a simple answer to this, but I have not thought of it yet. Where can I find some good examples that work with mapping sets?
Thanks!
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