How to load LAZY entities?
and how to make a property LAZY?
----------

Entity A has a reference to Entity B. (OneToOne)

@Entity
public class A {
// id ...

@OneToOne(fetch=FetchType.LAZY)
public B getB() {
return b;
}

public void setB(B b) {
this.b = b;
}


}

A a = aSessionBean.getA(id);
...

B b = a.getB();

org.hibernate.LazyInitializationException: could not initialize proxy - the owning Session was closed

when is the owning session created and closed?
What is the right way to access Lazy property(such as B above)?



-------

I tried to tag a property as LAZY using
@LOB
@Basic(fetch=FetchType.LAZY)
public String getDescription() {
...
}

it did not work either. It is being treated as EAGER from generated SQL.


Thank you for your help. Have a nice day.

Dave

1) You're using JBoss EJB3 right? so at your tx demarkation the session is opened and closed. You have to load the object within this tx either by accessing the object state or by using an ejbql fetch

2) Use the build time enhancement task as stated in the doc

I am using EJB3. It is not useful if I need to load the lazy entity in the same transaction. For web application, show some small data first, then show lazy(huge)data when user request it. they are in two transactions. If I use ejbql to load lazy data, will Hibernate build object relationship (navigation) that is defined using annotation such as @OneToMany ....

I am not quite understanding. I could not find doc related. A little more help is appreciated. thanks.

HA


and http://www.hibernate.org/hib_docs/v3/reference/en/html_single/#performance-fetching-lazyproperties

If you don't, then the ACID properties of your data will be lost which is *very* bad. To resume a work on an object graph wo loosing ACIDity, you can reattach it through the merge operation and use the optimistic locking (@Version).

code instrumentation is not a clean solution. My understanding is: If a property is declared to lazy, just do not include it in the select statement. It should be very simple. I do not like the way of instrumenting code. I think this is a bug. Delared to be lazy, but not lazy.

I have a class that has a Lob property that I like to be lazy. I can use query to select all properties except the lob. It is lazy. When I need the lob data, ask EM to load it. The problem is that, Hibernate does not support it in the case of association with other entities. For example,


class A {

@OneToMany(fetch=FetchType.EAGER)
public List<B> getBs();

...
}


class B {

@Basic(fetch=FetchType.Lazy)
@Lob(type=LobType.BLOB)
public getData();

}


When A is loaded, all the Bs will be loaded (eager). I like B's data not to be loaded. However Hibernate ignores Lazy.

One more question:

class A {

@ManyToMany(fetch=FetchType.LAZY)
@JoinTable (....)
public List<B> getBs();

...
}

In this case, it is lazy. Hibernate will not load Bs when loading A.

How to load Bs using EntityManager? I am not supposed to query JoinTable, right? Can EJB QL query join tables?

Thanks.

This is exactly what is done! But how do I know when you access a lazy property and that a second select has to be executed? By using byte code enhancement. The select and the bytecode enhancement are 2 orthogonal notions.


Wrong see above.



2 mistakes here.
@Lob(type=LobType.BLOB, fetch=FetchType.LAZY)
@Basic is not compatible with @Lob

Second a Blob if retrieved in a java.sql.Blob is already lazy as per the JDBC spec.

1. through navigation
2. using an ejb-ql query with the fetch keyword
select a from A a fetch join a.bs
3. by setting FetchType.EAGER

Please read the doc, this is explained.