JPA get foreign key without loading related record

Question

I am trying to optomize some reports and am trying to remove the need to load some expensive related records(multiple cascade loads, on each of ~500+ records).

My entity's are something like this:

@Entity
@Table(name="foo")
public class Foo
{
    @Id
    @Type(type = "pg-uuid")
    private UUID id;
...
    public UUID getId(){
        return id;
    }

and:

@Entity
@Table(name="bar")
public class Bar
{
    @Id
    @Type(type = "pg-uuid")
    private UUID id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name="FOO_ID")
    private Foo foo;
...

    public UUID getFooId(){
        return foo.getId();
    }

My issue is that when i make this call: return foo.getId(); hibernate goes and instantiates the Foo entity, and loads all related records even though i only want/need the id, which is a foreign key in my Bar table.

Is there any way to get Foo.Id from Bar without loading the Foo record? Thanks in advance.

Answer 1

Try mapping the FOO_ID column as a basic mapping instead or in addition to the existing Foo reference mapping:

@Entity
@Table(name="bar")
public class Bar
{
    @Id
    @Type(type = "pg-uuid")
    private UUID id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name="FOO_ID")
    private Foo foo;

    @Column(name="FOO_ID", insertable=false, updatable=false)
    private UUID fooId;
...

    public UUID getFooId(){
        return fooId;
    }

You can then decide to use fooId in queries to avoid joins. Because it is read-only, the FK value is still set from the foo reference - there may be some issues with a cache if you set the reference and not the fooId UUID; you can either set it yourself, or force a refresh to have JPA do it for you.

Answer 2

Hibernate thinks that getFooId() is a method that can potentially use any field of Bar . Hibernate can't check which fields exactly are used by the method, so it loads all lazy associations just in case.

You can use @Transient to avoid this

@Transient
public UUID getFooId(){
    return foo.getId();
}