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 Synony         


        

Answer 1

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.

Answer 2

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

Answer 3

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>();

Answer 4

I have achieved it via simply add

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

This help to remove duplicate.

Answer 5

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.

Answer 6

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,