Hibernate OneToOne automatic join fetching (resolving n+1 problem)?

If you want to shared primary keys between MainEntity and SubEntity use PrimaryKeyJoinColumn and MapsId annotation By using PrimaryKeyJoinColumn the entity is loaded by joining the MainEntity table with the SubEntity table using the same primary key. It should resolve the n+1 problems The MapsId annotation ask bernate to copy the identifier from another associated entity in our example will copy the SubEntity.mainEntity. Id to SubEntity.Id Entity public class MainEntity { @Id @GeneratedValue(strategy=GenerationType.

IDENTITY) @Column(name = "main_Id") private Long id; @OneToOne(cascade = CascadeType. ALL) @PrimaryKeyJoinColumn private SubEntity subEntity ; } @Entity public class SubEntity { @Id @Column(name="main_Id_FK") Long id; @MapsId @OneToOne @JoinColumn(name = "main_Id_FK") @PrimaryKeyJoinColumn private MainEntity mainEntity; } bernate Reference Documentation: PrimaryKeyJoinColumn MapsId.

If you want to shared primary keys between MainEntity and SubEntity use PrimaryKeyJoinColumn and MapsId annotation. By using PrimaryKeyJoinColumn the entity is loaded by joining the MainEntity table with the SubEntity table using the same primary key. It should resolve the n+1 problems.

The MapsId annotation ask bernate to copy the identifier from another associated entity in our example will copy the SubEntity.mainEntity. Id to SubEntity.id. @Entity public class MainEntity { @Id @GeneratedValue(strategy=GenerationType.

IDENTITY) @Column(name = "main_Id") private Long id; @OneToOne(cascade = CascadeType. ALL) @PrimaryKeyJoinColumn private SubEntity subEntity ; } @Entity public class SubEntity { @Id @Column(name="main_Id_FK") Long id; @MapsId @OneToOne @JoinColumn(name = "main_Id_FK") @PrimaryKeyJoinColumn private MainEntity mainEntity; } bernate Reference Documentation: PrimaryKeyJoinColumn MapsId.

I wasn't aware of the @MapsId, thanks for that. I'll try it out. Even in the case the n+1 problem would not be solved, I'd still prefer that approach to our current one.

Just a question: would that allow me to actually just set the id without having a reference to MainEntity? We have cases where missing SubEntity instances are created and we only have the id of the corresponding MainEntity. If possible, I'd like to rather not have to load MainEntity for just setting the reference.In other words: what would happen if id had a value and mainEntitywas null?

– Thomas Aug 12 at 8:21 I just had a look at the @MapsId documentation you linked, and unfortunately it seems that this annotation was introduced with 5 (and thus JPA 2). However, we're currently stuck with JBoss 4.2.3 (which doesn't support JPA 2) and thus can't use 5 (we're working on migration to JBoss 6, but that'll take a while). – Thomas Aug 12 at 12:21 @Thomas To Answer your first comment.

How to create a SubEntity with only the Id of the MainEntity. You need to use the EntityManager. GetReference to get a reference to MainEntity without loading it and set it to the SubEntiy.

Main relation without loading the Main. – Joël Hudon Aug 12 at 15:11 Without @MapsID you will have to manually assign the SubEntity.Id before saving it and set the SubEntity. Main relation insertable = false,updatable =false,nullable = false like in your example.

You can continue to use the @PrimaryKeyJoinColumn@ on MainEntity. SubEntity` this should correct the select problems. – Joël Hudon Aug 12 at 15:14 yes, I forgot about EntityManager.

GetReference(...) - thanks for reminding me. If I understand you correctly, I'd just have to use @PrimaryKeyColumn and leave the rest of the code unchanged, right? I'll try that and post the results.

– Thomas Aug 127 at 7:16.

There are three options to avoid the questions n +1: Lot size subselect Make a LEFT JOIN in the query Here FAQ1 Here FAQ2.

Additionally: Lot Size: I guess you mean Batch size - doesn't work here, subselect - I tried that but as already mentioned, it didn't work, left join - I'd rather do a fetch join, but as stated, I'd like to not have to add it to each query. – Thomas Aug 11 at 13:12.

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