Querydsl-如何在选择表达式中连接多个列的值？

Question

I'm using Querydsl with spring data jpa. I've implemented custom repository to find out person names. The person entity looks like as follows:

@Entity
@Table(name = "persons")
public class Person {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", nullable = false, updatable = false)
    private Long id;

    @Column(name = "initial")
    private String initial;

    @Column(name = "first_name")
    private String firstName;

    @Column(name = "last_name")
    private String lastName;

    @Column(name = "archived")
    private Boolean archived;

    //Other columns here.. && Getter setter goes here
}

The implemented custom repository:

@Repository
public class PersonRepositoryImpl extends QueryDslRepositorySupport 
        implements PersonRepositoryCustom {
    public PersonRepositoryImpl() {
        super(Person.class);
    }

    @Override
    public List<String> getPersonNames() {
        return getQuerydsl()
                .createQuery()
                .from(QPerson.person)
                .where(QPerson.person.archived.eq(Boolean.FALSE))
                .select(/* 
                    want to select as a full name like
                    initial + firstName + lastName
                  */)
                .fetch();
    }
}

In the select expression, I would like to join three columns to construct full name. If database has a record like the following -

id | initial | firstName | lastName | archived | ...
----------------------------------------------------
1  | "Mr"    | "John"    | "Snow"   | 0        | ...

then I would like to select as "Mr John Snow". Is there any way to do that?

I don't want to select whole record to concat the value because entity has others column and lots of associations and it's not good to load whole record.

Answer 1

You can select only the 3 columns you need by using 3.2.3. Constructor usage from Querydsl documentation.

This way you will need to join them programmatically, but you will achieve to select only the 3 columns you need.

Create a class that has only the fields you need.

public class PersonDTO {
    private String initial;
    private String firstName;
    private String lastName;

    public PersonDTO(String initial, String firstName, String lastName) {
        this.initial = initial;
        this.firstName = firstName;
        this.lastName = lastName;
    }
}

Then you can create the query

           query.from(QPerson.person)
                .select(Projections.constructor( 
                        PersonDTO.class, 
                        QPerson.person.initial, 
                        QPerson.person.firstName, 
                        QPerson.person.lastName))
                .fetch();

Answer 2

@chris-sekas provide a good solution, but here is another one that seems to be very handy. I've used com.querydsl.core.Tuple instead of creating new DTO.

QPerson person = QPerson.person;
List<Tuple> tuples = getQuerydsl()
        .createQuery()
        .from(person)
        .where(person.archived.eq(Boolean.FALSE))
        .select(person.initial, 
                person.firstName,
                person.lastName)
        .fetch();

List<String> names = tuples.stream()
        .map(t ->
                t.get(person.initial) +
                " " +
                t.get(person.firstName) +
                " " +
                t.get(person.lastName))
        .distinct()
        .collect(Collectors.toList());

It's very useful when we need few column's information only and don't need to create new DTO.