Hibernate CollectionOfElements EAGER fetch duplicates elements

Question

I have a class called SynonymMapping which has a collection of values mapped as a CollectionOfElements

@Entity(name = "synonymmapping")
public class SynonymMapping {

    @Id private String keyId;

    //@CollectionOfElements(fetch = FetchType.EAGER)
    @CollectionOfElements
    @JoinTable(name="synonymmappingvalues", joinColumns={@JoinColumn(name="keyId")})
    @Column(name="value", nullable=false)
    @Sort(type=SortType.NATURAL)
    private SortedSet<String> values;

    public SynonymMapping() {
        values = new TreeSet<String>();
    }

    public SynonymMapping(String key, SortedSet<String> values) {
        this();
        this.keyId = key;
        this.values = values;
    }

    public String getKeyId() {
        return keyId;
    }

    public Set<String> getValues() {
        return values;
    }
}

I have a test where I store two SynonymMapping objects to the database and then ask the database to return all saved SynonymMapping objects, expecting to receive the two objects I stored.

When I change the mapping of values to be eager (as shown in in the code by the commented out line) and run the test again, I receive four matches.

I have cleared out the database between runs and I can duplicate this problem swapping between eager and lazy.

I think it has to do with the joins that hibernate creates underneath but I can't find a definite answer online.

Can anyone tell me why an eager fetch is duplicating the objects?

Thanks.

Every one with the Exception "More than one row with the given identifier was found" should know about this. It really spares alot of hours not knowing what the hell is going wrong. See @user176668 answer!! — Gotham Llianen, May 27 '16 at 10:31

score 80 · Answer 1 · answered Sep 21 '09 at 16:46

80

I stepped into the same problem - when you set the FetchType.EAGER for a @CollectionOfElements, the Hibernate tries to get everything in one shot, i.e. using one single query for each entry of element linked to a "master" object. This problem can be successfully solved at a cost of N+1 query, if you add the @Fetch (FetchMode.SELECT) annotation to your collection. In my case I wanted to have a MediaObject entity with a collection of its metadata items (video codec, audio codec, sizes, etc.). The mapping for a metadataItems collection looks as follows:


@CollectionOfElements (targetElement = String.class, fetch = FetchType.EAGER)
@JoinTable(name = "mo_metadata_item", joinColumns = @JoinColumn(name = "media_object_id"))
@MapKey(columns = @Column(name = "name"))
@Column (name = "value")
@Fetch (FetchMode.SELECT)
private Map<String, String> metadataItems = new HashMap<String, String>();

answered Sep 21 '09 at 16:46

1

I liked the solution, because we dont need to wrap the result with a Set to get uniqueness. – sanbhat Jan 08 '13 at 22:00
can it also be caused by caching such as EHcache? – pete Feb 24 '15 at 23:10
This solution works great, however, it's implementation-dependent. – rion18 Apr 28 '15 at 19:25
1

Thank you. I have done a lot search about it. Every body said to use Set. – mirzaei Oct 15 '16 at 08:13
It works for me. Too bad @Fetch is not a JPA annotation, but who cares. I'm wondering what you are supposed to do when using pure JPA though. It's a bit frustrating. – xtian Feb 06 '18 at 16:23
2

Took ages to debug this, and even longer to find an answer that wasn't a tutorial on implementing collections. Thanks for sharing. – AndyT Sep 27 '18 at 14:14
@user176668 Why you don't use SUBSELECT instead of SELECT? – mgh May 13 '23 at 10:08

score 34 · Accepted Answer · answered Jul 07 '09 at 17:34

34

It's generally not a good idea to enforce eager fetching in the mapping - it's better to specify eager joins in appropriate queries (unless you're 100% sure that under any and all circumstances your object won't make sense / be valid without that collection being populated).

The reason you're getting duplicates is because Hibernate internally joins your root and collection tables. Note that they really are duplicates, e.g. for 2 SynonymMappings with 3 collection elements each you would get 6 results (2x3), 3 copies of each SynonymMapping entity. So the easiest workaround is to wrap results in a Set thereby ensuring they're unique.

answered Jul 07 '09 at 17:34

ChssPly76

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But why can Hibernate not filter these out, I cannot see why you would ever want it this this like this. – Paul Taylor Feb 28 '12 at 17:20
I can confirm it works. I was using a Collection just in general, and that was a mistake. – Aron Lorincz Jun 22 '15 at 08:39
Note: wrapping in a Set will render incorrect any pagination done on the results – Sandy Simonton Jul 09 '21 at 18:22
Is it in general good habit to use Set<>? It solves this "duplicate" problem so I'd use it for all my collections. Is there any downside of it? – dpelisek Aug 10 '21 at 10:14
@dpelisek Well, in my case, switching from a List to a Set made it _worse_: I went from “all elements in three copies” to _nine_ copies. – Alice M. Jun 30 '22 at 17:43
@SandySimonton, your comment is irrelevant, because initial issue was about "subset" of objects, and if you are talking about pagination, most probably you need to work with "subobjects" as a "root". – degr May 01 '23 at 11:12
@Alice-m, it possible only in case when you did not redefine equals and hashcode. Otherwise check what kind of collection is used for your set. – degr May 01 '23 at 11:12
@dpelisek, yes, sure, objects order is lost. – degr May 01 '23 at 11:14

score 8 · Answer 3 · answered May 26 '16 at 08:12

8

I have faced this problem and I solved it using

criteria.setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY);

This clears out the duplicates which are caused by the join made to the child tables.

answered May 26 '16 at 08:12

Raja Anbazhagan

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Arthur Ronald · Answer 4 · 2010-01-07T04:48:31.600

2

You could use a SELECT DISTINCT (Hibernate Query Language) clause as follows

SELECT DISTINCT synonym FROM SynonymMapping synonym LEFT JOIN FETCH synonym.values

DISTINCT clause removes duplicate references in Hibernate.

Although both component and value-type collection has its lifecycle bound to the owning entity class, you should declare them in select clause in order to retrieve them. (LEFT JOIN FETCH synonym.values)

ChssPly76's answer is another approach, but does not forget override equals and hashcode method according to Set semantic

regards,

edited Jan 07 '10 at 04:48

answered Jul 17 '09 at 13:50

Arthur Ronald

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I was wondering more about why this happens and why the design decision was taken for hibernate to respond like this more than the different ways to get around it. – Rachel Jul 20 '09 at 12:45

score 0 · Answer 5 · answered Sep 02 '17 at 12:34

Instead of FetchMode.SELECT with N+1 queries it is better using BatchSize e.q. @BatchSize(size = 200).

DISTINCT and Criteria.DISTINCT_ROOT_ENTITY doesn't help, if you have to fetch more than 1 association. For this case see other solutions: https://stackoverflow.com/a/46013654/548473

score 0 · Answer 6 · answered Aug 29 '18 at 13:07

0

I have achieved it via simply add

session.createCriteria(ModelClass.class).setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY);

This help to remove duplicate.

answered Aug 29 '18 at 13:07

Rahul Gangahar

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Hibernate CollectionOfElements EAGER fetch duplicates elements

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