jpa avoid query - jpa

Question

i have the next class
#Entity
#Table(name = "table_order")
#IdClass(OrderPK.class)
public class Order {
/** Unique identifier of the currency code in which the transaction was negociated. */
#Column(name = "TRADECURRE", nullable = false, length = 5)
private String tradeCurrencyCode;
/** Currency Entity for Trade. */
#ManyToOne(optional = true, fetch = FetchType.LAZY)
#JoinColumns({
#JoinColumn(name = "TRADECURRE", referencedColumnName = "codigo", updatable = false, insertable = false) })
private Currency currencyEntity;
.. here get and sets
}
then execute the next query:
StringBuilder jpaQuery = new StringBuilder();
StringBuilder whereClause = new StringBuilder();
jpaQuery.append("SELECT o, o.currencyEntity");
List orders = query.getResultList();
in this point the log of jpa show 2 querys executed, one to order table and other to Currency table.
bellow i write the next code (in the same class and method of the previous code)
for (Object orderElement : orders) {
int indexArray = 0;
Object[] orderArray = (Object[]) orderElement;
Order orderEntity = (Order) orderArray [indexArray++];
orderEntity.setCurrencyEntity((Currency) orderArray [indexArray++]);
}
When the line
orderEntity.setCurrencyEntity((Currency) orderArray [indexArray++]);
is executed, the query over the table currency is executed once again at database. I need avoid this query to fix some performance problems, i have all the data in the orderArray.
i'm using eclipselink 1.1
thanks in advance

Answer 1

This is happening because you haven't told JPA to pre-fetch the currentEntity in the initial select (although I think that's what you were trying to do with SELECT o, o.currencyEntity). As a result, JPA has to fetch the currentEntity each time round the loop, and it's a real performance killer.
The way to do this with JPA is with a fetch join (documented here). You'd write your query like this:
SELECT o from Order o LEFT JOIN FETCH o.currencyEntity
This also makes it easier to navigate the result set than with SELECT o, o.currencyEntity, since you'll only have a single entity returned, with its currencyEntity property intact:
List<Order> orders = query.getResultList();
for (Order order : orders) {
// fully-populated, without requiring another database query
Currency ccy = order.getCurrentEntity();
}