How to left join fetch multiple children in Hibern

问题:

I'm working with hibernate and I'm having troubles creating an hql query that fetches all the children of my object.

For example: The Object User has a list of Cars and a list of Friends.

To get a user with his cars I would use following query:

from User u left join fetch u.cars where u.id = ?

This works fine, so I thought it would be easy to get a user with his cars and with his friends with following query:

from User u left join fetch u.cars left join fetch u.friends where u.id = ?

But this gives me following error:

HibernateException: cannot simultaneously fetch multiple bags

Now my question is: what is the right way to fetch multiple children in hibernate?

回答1:

At most one of the children collection must be a bag (i.e. declared as a List). Declare the other collections as Sets, and it will work.

Beware, though, that doing such fetch joins makes a cartesiann product of the rows. If both collections have 100 elements, such a query retrieves 10,000 rows fro the database. It's sometimes more efficient to execute a first query which fetches one collection, and a second one which fetches the other (which thus reduces the number of rows retrieved to 200). This is also a way to avoid the problem you have:

select u from User u left join fetch u.cars where u.id = :id;
select u from User u left join fetch u.friends where u.id = :id;

回答2:

You just hit the Collection/List (bag) issue.

Here is a link to an Hibernate official "issue" about this: https://hibernate.atlassian.net/browse/HHH-1718. As you can see, it has been open in 2006 and is still open.

In addition to what JB Nizet propose, I suggest you use Set instead of Collection or List in your model (if you can), otherwise, you can also specify your Collection/List as FetchMode.SUBSELECT and as a last option (painful to implement), you can use @IndexColumn on your @OneToMany/@ManyToMany.

This blog article will guide you into implementing solutions: http://jroller.com/eyallupu/entry/hibernate_exception_simultaneously_fetch_multiple

In other words, there is no solution to do exactly what you want to do, except workarounds.

Hope this helps!

Edit: typo

回答3:

Not possible as it would be too heavy for the application/database, you need to create 2 separate criteria and retrieve data separately.

Cat cat = sess.createCriteria(Cat.class)
              .add(Restrictions.like("name", "F%"))
              .uniqueResult();

List kitten = sess.createCriteria(Kitten.class)
                  .add(Restrictions.eq("cat", cat))
                  .createCriteria("kittens")
                  .add(Restrictions.like("name", "F%"))
                  .list();

List mate = sess.createCriteria(Mate.class)
                .add(Restrictions.eq("cat", cat))
                .createCriteria("mate")
                .add(Restrictions.like("name", "F%"))
                .list();