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Why does JOIN FETCH return duplicates?

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marcelru

Post subject: Why does JOIN FETCH return duplicates?

Posted: Wed Aug 24, 2016 9:06 am

Newbie

Joined: Wed Aug 24, 2016 8:52 am
Posts: 1

Hi,

I'm using hibernate-5.2.1-JPA2.1 via openejb-core-7.0.1 in Java SE 8 with Postgresql 9.5.4 env.

I have a 1 (BuddyRelation) :n (BuddyPermission) Relation

The query

Code:

SELECT br FROM BuddyRelation br LEFT JOIN FETCH br.buddyPermissions WHERE br.account.accountId = :accountId

returns not only the distinct BuddyRelations but a row for each 1:n BuddyPermission.
I would have expected that only the distinct BuddyRelation entries are returned, with properly filled buddyPermissions Map.

Code:

class BuddyRelation
   ...
   @OneToMany(mappedBy = "buddyRelation", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
   @OrderBy("permission ASC")
   @MapKey(name = "permission")
   @OnDelete(action = OnDeleteAction.CASCADE)
   public Map<String, BuddyPermission> getBuddyPermissions() {
      return buddyPermissions;
   }
  ...
}

What is wrong?

Thanks
Marcel

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vlad

Post subject: Re: JOIN FETCH returns too many rows?

Posted: Mon Aug 29, 2016 5:54 am

Hibernate Team

Joined: Thu Sep 11, 2014 2:50 am
Posts: 1628
Location: Romania

Because the underlying JDBC ResultSet contains one BuddyRelation entity for each BuddyPermission being fetched, so does the List<BuddyRelation> will contain duplicates. You need to use DISTINCT to remove those duplicates from the entity List:

Code:

SELECT DISTINCT br FROM BuddyRelation br LEFT JOIN FETCH br.buddyPermissions WHERE br.account.accountId = :accountId

You can upgrade to 5.2.2, and make use of the [url=new http://in.relation.to/2016/08/04/introd ... uery-hint/]DISTINCT pass-through Hibernate Query Hint[/url].

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