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

Question

we have a n+1 select problem with Hibernate 3.3.

For simplicity's sake, I'll just do a short abstract example.

Suppose we have the following simple classes:

class MainEntity {
  @Id
  public Long id; //we have a table generator create this id

  @OneToOne ( mappedBy ="main" )
  public SubEntity subEntity;
}

class SubEntity {
 @Id
 @Column( name = "mainId" ) //note that this is the same column as the join column below
 public Long mainId; //in order to have the exact same id as the corresponding MainEntity

 @OneToOne ( fetch = FetchType.LAZY )
 @JoinColumn ( name = "mainId", insertable = false, updatable = false, nullable = false )
 public MainEntity main; //this is used for navigation and queries (" ... subentity.main = :x")
}

So as you can see SubEntity has a relation to MainEntity that is expressed by two properties, where the mainId property is the one responsible for managing the relation/foreign key.

This works quite well and perfectly fits our needs.

However, there's one problem with eagerly loading the SubEntity along with the MainEntity.

Suppose I have a query that returns a collection of MainEntity. With the current setup, Hibernate will issue n + 1 selects: the query itself + n selects for each SubEntity.

Of course I could add a join fetch to the query, but I'd rather like Hibernate to do that automatically. Thus I tried adding @Fetch( FetchMode.JOIN ), but that didn't do anything.

I would also have no problem using @Fetch( FetchMode.SUBSELECT ), which should reduce the select statements to 2 - the original query and a select for the sub entities (at least that's what happens on another property annotated with @CollectionOfElements and @Fetch( FetchMode.SUBSELECT )).


So the question is: how would I tell Hibernate to automatically join fetch or use a single select in order to eagerly load the sub entities? Am I missing something?

Thanks in advance,

Thomas

PS: One thing that might be a problem might be the mappedBy = "main" which doesn't reference the actual id column, but I can't change it to mappedBy = "id".

Answer 1

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 Hibernate 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;        

}

Hibernate Reference Documentation:

PrimaryKeyJoinColumn
MapsId

Answer 2

There are three options to avoid the questions n +1:

 Lot size

 subselect

 Make a LEFT JOIN in the query

Here FAQ1 Here FAQ2