如何在 QueryDSL 中创建一个没有实体或与两个表关联的 QClass

Question

let's say that I have three classes

    @Entity
    public class Book {
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private Long id;

        @Column
        private String name;
    }

    @Entity
    public class Author {
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private Long id;

        @Column
        private String fistName;

        @Column
        private String lastName;
    }

    @Entity
    public class AuthorBookRelation {
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private Long id;

        @ManyToOne
        @JoinColumn(name = "author", referencedColumnName = "id")
        private Author author;

        @ManyToOne
        @JoinColumn(name = "book", referencedColumnName = "id")
        private Book book;
    }

And accordingly, I have three Qclasses

QAuthor QBook QAuthorBookRelation.

but I can do just two classes.

@Entity
    public class Book {
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private Long id;

        @Column
        private String name;
    }

    @Entity
    public class Author {
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private Long id;

        @Column
        private String fistName;

        @Column
        private String lastName;

        @ManyToMany
        @JoinTable(
                name = "author_book_relation",
                joinColumns = {@JoinColumn(name = "author")},
                inverseJoinColumns = {@JoinColumn(name = "book")}
        )
        private List<Book> books = new ArrayList<>();
    }

Then there is not need for class AuthorBookRelation. But i need QAuthorBookRelation to create a request with queryDSL

How to create a class that is bound to two tables?

Answer 1

I think you're missing the point of JPA, JPQL and QueryDSL. You should join associated entities. There is absolutely no need for association entities in any of these technologies, unless you need to associate data with this association.

Otherwise, you'd just do:

List<Tuple> result = query().from(QBook.book)
     .innerJoin(QBook.book.author, author)
     .select(QBook.book, QAuthor.author)
     .fetch();

Which is equivalent to the following JPQL:

SELECT book, author FROM Book book INNER JOIN book.author author