OpenJPA fetching entity with a lazy collection

Question

I have two entities :

A:

class A {

 @Id
 Long id;

 @OneToMany(fetch = FetchType.LAZY)
 List<B> listOfB;
}

and class B :

class B {

 @Id
 Long id;

 @ManyToOne(fetch = FetchType.LAZY)
 A a;
}

now in my spring data repo I'm creating a query like :

@Query("SELECT a FROM A a INNER JOIN a.listOfB b WHERE b.id = :id")

The problem is, that the query is executed and returns some A objects, but when I want to access the listOfB I'm getting a NullPointerException ...

A a = aRepository.findByOwnQuery(id);
a.getListOfB().size(); -> NullPointerException

Post your code, and post the exact and complete stack trace of the exception. — JB Nizet, Aug 14 '17 at 10:56
What code do you need ? There'are two entities with relation marked as lazy, and a query. — Bart, Aug 14 '17 at 10:59
so the list field is NULL, and the object "a" is presumably detached by that point so cannot fetch the field from the database. So fetch it before unmanaging the "a" object — Neil Stockton, Aug 14 '17 at 13:27
@NeilStockton how ? I assume that when I'm joining the B class by `INNER JOIN a.listOfB b` I will have this list in result object ... — Bart, Aug 14 '17 at 13:55
nope. that simply joins to form the query. "JOIN FETCH" is valid JPQL syntax ... — Neil Stockton, Aug 14 '17 at 13:59

Abdullah Khan · Answer 1 · 2017-08-14T11:10:25.720

-1

The NullPointerException could be because you are fetching your child listOfB Lazily in your parent class A.

Try JOIN FETCH like so

@Query("SELECT a FROM A a JOIN FETCH a.listOfB b WHERE b.id = :id")

edited Aug 14 '17 at 11:10

answered Aug 14 '17 at 11:04

Abdullah Khan

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With such contruction I'm getting the `ArgumentException` error `Encountered "b" at ... but expected: [",", "GROUP", "HAVING", "INNER", "JOIN", "LEFT", "ORDER", "WHERE", ]`. – Bart Aug 14 '17 at 11:43
Are you sure this exception is because of the single change i suggested.?Because my change doesn't change the argument. It only changes the strategy to fetch to children. Please cross check. – Abdullah Khan Aug 14 '17 at 11:50
Yep, because the previous query executes fine but returns null for the list. – Bart Aug 14 '17 at 11:53
3

The JPA spec doesn't allow use the 'b' defined on a fetch join clause within the query. You would have to create a separate join for the query to use in the where clause. – Chris Aug 14 '17 at 17:41
@Chris Thats something i didn't know. Can you post some reference. – Abdullah Khan Aug 15 '17 at 06:04
I don't have a link to the spec, but the definition in "4.4.5.3 Fetch Joins" of jpa 2.1 does not include aliases. Its posted in other answers elsewhere as well though https://stackoverflow.com/a/16693590/496099 – Chris Aug 15 '17 at 18:44

OpenJPA fetching entity with a lazy collection

1 Answers1