java - Hibernate Criteria returns children multiple times with FetchType.EAGER

Question

I have an Order class that has a list of OrderTransactions and I mapped it with a one-to-many Hibernate mapping like so:

@OneToMany(targetEntity = OrderTransaction.class, cascade = CascadeType.ALL)
public List<OrderTransaction> getOrderTransactions() {
    return orderTransactions;
}

These Orders also have a field orderStatus, which is used for filtering with the following Criteria:

public List<Order> getOrderForProduct(OrderFilter orderFilter) {
    Criteria criteria = getHibernateSession()
            .createCriteria(Order.class)
            .add(Restrictions.in("orderStatus", orderFilter.getStatusesToShow()));
    return criteria.list();
}

This works and the result is as expected.

Now here is my question: Why, when I set the fetch type explicitly to EAGER, do the Orders appear multiple times in the resulting list?

@OneToMany(targetEntity = OrderTransaction.class, fetch = FetchType.EAGER, cascade = CascadeType.ALL)
public List<OrderTransaction> getOrderTransactions() {
    return orderTransactions;
}

How would I have to change my Criteria code to reach the same result with the new setting?

Answer 1

This is actually the expected behaviour if I understood your configuration correctly.

You get the same Order instance in any of the results, but since now you are doing a join with the OrderTransaction, it has to return the same amount of results a regular sql join will return

So actually it should apear multiple times. this is explained very well by the author (Gavin King) himself here: It both explains why, and how to still get distinct results

---

Also mentioned in the Hibernate FAQ :

Hibernate does not return distinct results for a query with outer join fetching enabled for a collection (even if I use the distinct keyword)? First, you need to understand SQL and how OUTER JOINs work in SQL. If you do not fully understand and comprehend outer joins in SQL, do not continue reading this FAQ item but consult a SQL manual or tutorial. Otherwise you will not understand the following explanation and you will complain about this behavior on the Hibernate forum.

Typical examples that might return duplicate references of the same Order object:

List result = session.createCriteria(Order.class)
                    .setFetchMode("lineItems", FetchMode.JOIN)
                    .list();

---

<class name="Order">
    ...
    <set name="lineItems" fetch="join">

---

List result = session.createCriteria(Order.class)
                       .list();
List result = session.createQuery("select o from Order o left join fetch o.lineItems").list();

All of these examples produce the same SQL statement:

SELECT o.*, l.* from ORDER o LEFT OUTER JOIN LINE_ITEMS l ON o.ID = l.ORDER_ID

Want to know why the duplicates are there? Look at the SQL resultset, Hibernate does not hide these duplicates on the left side of the outer joined result but returns all the duplicates of the driving table. If you have 5 orders in the database, and each order has 3 line items, the resultset will be 15 rows. The Java result list of these queries will have 15 elements, all of type Order. Only 5 Order instances will be created by Hibernate, but duplicates of the SQL resultset are preserved as duplicate references to these 5 instances. If you do not understand this last sentence, you need to read up on Java and the difference between an instance on the Java heap and a reference to such an instance.

(Why a left outer join? If you'd have an additional order with no line items, the result set would be 16 rows with NULL filling up the right side, where the line item data is for other order. You want orders even if they don't have line items, right? If not, use an inner join fetch in your HQL).

Hibernate does not filter out these duplicate references by default. Some people (not you) actually want this. How can you filter them out?

Like this:

Collection result = new LinkedHashSet( session.create*(...).list() );