I have the following entities: Event and Attribute. An event can have many attributes.
The problem I am having is, I can't seem to find a query that returns the events with their attributes in a single SQL query. I could have millions of events that need to be iterated through and I don't want to lazy load them all for obvious performance reasons.
I've tried using fetch in the query, but it returns an event for every attribute. ie if an event had 2 attributes it would return 2 events each with one attribute.
What I want is one event with 2 attributes.
SELECT e FROM Event e LEFT JOIN FETCH e.attributes
If I add DISTINCT it works, but then it creates a distinct SQL query which is extremely slow over large data sets.
public class Event {
@OneToMany(mappedBy = "event", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
public Set<Attribute> getAttributes(){}
}
public class Attribute {
@ManyToOne(cascade=CascadeType.ALL)
@JoinColumn(name = "event_id", nullable = false)
public Event getEvent() {
return event;
}
}
Solution
I couldn't find a JPA solution. Instead, I unwrapped the EntityManager to the Hibernate session and used the criteria's result transformer as @milkplusvellocet suggested. This retrieves distinct root entities without creating a distinct SQL query.
Session session = em.unwrap(Session.class);
Criteria criteria = session.createCriteria(Event.class);
criteria.setFetchMode("attributes", FetchMode.JOIN);
criteria.setResultTransformer(CriteriaSpecification.DISTINCT_ROOT_ENTITY);
One thing to note is, I tried using HQL from the unwrapped session. I added DISTINCT to the query and set the result transformer to DISTINCT_ROOT_ENTITY similar to the criteria solution above. Doing it this way still created a distinct SQL query.
You need a ResultTransformer
.
Try the following HQL:
SELECT DISTINCT e FROM Event e LEFT JOIN FETCH e.attributes
This is equivalent to using CriteriaSpecification.DISTINCT_ROOT_ENTITY
in a criteria query.
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