Question

This is a related question to another question:

Hibernate exception PropertyNotFoundException when using Transformer

I have several tables joining hql. The tables are like this:

A
- A_ID
- NAME

B
- B_ID
- A_ID

C
- C_ID
- B_ID
- LENGTH
- UNIT

Classes:

@Entity
@Table(name="A")
class A
{
    @Id
    @Column(name="A_ID", updatable=false)
    private Long id;

    @Column(name="NAME", nullable=false, length=10, updatable=false)
    private String name;

    @OneToMany(mappedBy="a", fetch=FetchType.LAZY, cascade={CascadeType.ALL})
    @JoinColumn(name="A_ID", nullable=false)
    private Set<B> bs;

    // Setters and getters
    ...
}

@Entity
@Table(name="B")
class B
{
    @Id
    @Column(name="B_ID", updatable=false)
    private Long id;

    @ManyToOne
    @JoinColumn(name="A_ID", nullable=false, insertable=true, updatable=false)
    private A a;

    @OneToMany(mappedBy="b", fetch=FetchType.LAZY, cascade={CascadeType.ALL})
    @JoinColumn(name="B_ID", nullable=false)
    private Set<C> cs;

    @Transient
    private Double length;

    @Transient
    private String unit;

    // Setters and getters
    ...
}

@Entity
@Table(name="C")
class C
{
    @Id
    @Column(name="C_ID", updatable=false)
    private Long id;

    @ManyToOne
    @JoinColumn(name="B_ID", nullable=false, insertable=true, updatable=false)
    private B b;

    @Column(name="LENGTH", nullable=false, updatable=false)
    private Double length;

    @Column(name="UNIT", nullable=false, length=10, updatable=false)
    private String unit;

    // Setters and getters
    ...
}

After solving that issue, it is like this now:

select b.id as id, sum(c.length) as length, min(c.unit) as unit
from B b
left outer join b.c as c
group by b.id

Now the problem is:

I don’t know how to set the alias of A in the returned B object in the HQL. This is causing a NPE when I’m doing this:

b.getA().getName();

Because I don’t know how to set the related object “A” with “B” since there is only an ID in the B table.

Please help. Thank you very much.

Answer 1

b.getA().getName(); This will throw a NPE because select b.id as id, sum(c.length) as length, min(c.unit) as unit from B b does not include A. When you use the select clause in a HQL query it will only return the fields mentioned.

Please refer to http://docs.jboss.org/hibernate/orm/3.3/reference/en/html/queryhql.html for more information.

EDIT:

If you read through the documentation provided you would have see select cat.mate from Cat cat. This can be incorporated to your query. Add b.A in your select clause and it should work.

EDIT:

If select b.id as id, sum(c.length) as length, min(c.unit) as unit
from B b
left outer join b.c as c
group by b.id
work then
select b.id as id, sum(c.length) as length, min(c.unit) as unit, b.A
from B b
left outer join b.c as c
group by b.id, b.A
should as well.