Hi,

I've been reading the JPA spec lately and have come across a trouble understanding how LEFT JOIN FETCH is supposed to work.

Let's assume I have two entities - Person and Project. Let's assume that there's a @ManyToMany association between Person and Project where the attribute - projects - of Person points to the Projects the person is involved into.

Provided that there're 2 people (2 Person entities) and 3 projects in a database where only the person X is assigned to 3 projects.

The following query will return 4 Person entities whereas only 2 are unique (distinct).

Query query = em

.createQuery("SELECT DISTINCT p FROM Person p LEFT JOIN FETCH p.projects");

List<Person> people = query.getResultList();

assert people.size() == 4;

Set<Person> peopleSet = new LinkedHashSet(people);

assert peopleSet.size() == 2;

Why am I getting 4? Shouldn't a JPA provider return only 2, esp. when DISTINCT is used? Or if it's a default behaviour to return 4, how to filter out the others unneeded? Is there a JPA property? Is java.util.Set the only solution to it? What about the penalty of instantiating each and every Person instance when only 2 are really of use?

Just for the record, I'm using Apache OpenJPA 0.9.7-SNAPSHOT and Apache Derby.

Jacek

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Jacek Laskowski

http://www.JacekLaskowski.pl