QueryDSL: Unsupported expression sum

Question

I have this Bean object:

@Entity
@Table(name="a_table")
public class A {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;

@Column(name="amount")
private int amount;

@ManyToOne
private B b;

@ManyToOne
private C c;

public MunicipalityInventory(int amount, B b, C c) {
    this.amount = amount;
    this.b = b;
    this.c = c;
}

I try to bind my query result to object above. My first try with QueryDSL was something like this (it worked in that sense that the presentation of data was correct, but it didn't bind property's of query to the object, because it returned Tuple which contained StringPath, NumberExpression and other weird data types for me)

public List<Tuple> findAll(){
    QA a = QA.A;
    QB b = QB.b;
    QC c = QC.c;

    JPAQuery query = new JPAQuery(entityManager);

    return query.from(a)
            .innerJoin(a.b, b)
            .innerJoin(a.c, c)
            .orderBy(c.name.asc())
            .groupBy(c.name, a.c, c.name)
            .list(a.amount.sum(), b.name, c.name);
}

As I said this worked, but now I am trying to convert that example to this (oh, and that new condition in groupBy was needed to get this compile) so I can work right away with proper object:

    return query.from(a)
            .innerJoin(a.b, b)
            .innerJoin(a.c, c)
            .orderBy(b.name.asc())
            .groupBy(c.name, a.amount, b.id, c.id)
            .list(Projections.fields(A.class, a.amount.sum(), b, c));

When compiling this it will give error warning

java.lang.IllegalArgumentException: Unsupported expression sum(a.amount)
    at com.mysema.query.types.QBean.createBindings(QBean.java:70)

Why's that? For my understanding this should work.

Answer 1

Querydsl can't determine from the sum() expression which field of A you want to populate.

Use an alias to define the target field:

a.amount.sum().as("fieldName")

Querydsl can handle only paths and alias expressions when binding to fields or properties.