How do I left join tables in unidirectional many-to-one in Hibernate?

Question

I'm piggy-backing off of http://stackoverflow.com/questions/2368195/how-to-join-tables-in-unidirectional-many-to-one-condition.

If you have two classes:

class A {
    @Id
    public Long id;
}

class B {
    @Id
    public Long id;
    @ManyToOne
    @JoinColumn(name = "parent_id", referencedColumnName = "id")
    public A parent;
}

B -> A is a many to one relationship. I understand that I could add a Collection of Bs to A however I do not want that association.

So my actual question is, Is there an HQL or Criteria way of creating the SQL query:

select * from A left join B on (b.parent_id = a.id)

This will retrieve all A records with a Cartesian product of each B record that references A and will include A records that have no B referencing them.

If you use:

from A a, B b where b.a = a

then it is an inner join and you do not receive the A records that do not have a B referencing them.

I have not found a good way of doing this without two queries so anything less than that would be great.

Thanks.

Answer 1

You may do so by specifying the fetch attribute.

(10) fetch (optional) Choose between outer-join fetching and fetching by sequential select.

You find it at: Chapter 6. Collection Mapping, scroll down to: 6.2. Mapping a Collection

EDIT

I read in your question's comment that you wanted a way to perform a raw SQL query? Here a reference that might possibly be of interest:

Chapter 13 - Native SQL Queries

and if you want a way to make it possible through HQL:

Chapter 11. HQL: The Hibernate Query Language

In chapter 11, you want to scroll down to 11.3. Associations and joins.

IQuery q = session.CreateQuery(@"from A as ClassA left join B as ClassB");

I guess however that ClassB needs to be a member of ClassA. Further reasdings shall help.

Another thing that might proove to be useful to you are named queries:

<query name="PeopleByName">
from Person p
where p.Name like :name
</query>

And calling this query from within code like so:

using (var session = sessionFactory.OpenSession())
    using (var tx = session.BeginTransaction()) {
        session.GetNamedQuery("PeopleByName")
            .SetParameter("name", "ayende")
            .List();
        tx.Commit();
    }

Please take a look at the referenced link by Ayende who explains it more in depth.

Answer 2

I've made an example with what you posted and I think this may work:

select a,b from B as b left outer join b.parent as a in HQL.

I have to find a "criteria" way of doing that though.