java - 使用 Transformer 将实体映射到 DTO 时获取实体内的集合

Question

我有一个名为 Student 的实体

@Entity
@Table(name = "students")
public class Student implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "STUDENT_ID")
    private Integer studentId;

    @Column(name = "STUDENT_NAME", nullable = false, length = 100)
    private String studentName;

    @OneToMany(fetch = FetchType.EAGER, mappedBy = "student", cascade = CascadeType.ALL)
    private List<Note> studentNotes;

    // Some other instance variables that are not relevant to this question

    /* Getters and Setters */

}

和一个名为 Note 的实体

@Entity
@Table(name = "notes")
public class Note implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "NOTE_ID")
    private Integer noteId;

    @Column(name = "NOTE_CONTENT")
    private String noteText;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "STUDENT_ID")
    private Student student;

    /* Getters and Setters */

}

正如您所看到的，该关系表明学生可以拥有多个笔记。

为了在特定页面上显示有关学生的一些信息，我只需要学生姓名、笔记计数和所有笔记。我为此创建了一个 StudentDTO，它看起来像这样:

public class StudentDTO {

    private Long count;
    private String name;
    private List<Note> notes;

    /* Getters and setters */

}

我使用以下代码将从数据库返回的 Student 和 Notes 映射到 StudentDTO

private static void testDTO() {
        Session session = getSessionFactory().openSession();
        String queryString = "SELECT count(n) as count, s.studentName as name, s.studentNotes as notes " +
                "from Student s join s.studentNotes n where s.id = 3";
        Query query = session.createQuery(queryString);

        List<StudentDTO> list = query.setResultTransformer(Transformers.aliasToBean(StudentDTO.class)).list();
        for (StudentDTO u : list) {
            System.out.println(u.getName());
            System.out.println(u.getCount());
            System.out.println(u.getNotes().size());
        }
    }

当查询中获取注释时，上述代码会失败，但如果我删除注释并仅获取名称和计数，则它可以正常工作。

当查询中包含注释时，Hibernate 会引发以下错误:

select
            count(studentnot2_.NOTE_ID) as col_0_0_,
            . as col_3_0_,
            studentnot3_.NOTE_ID as NOTE_ID1_2_,
            studentnot3_.NOTE_CONTENT as NOTE_CON2_2_,
            studentnot3_.STUDENT_ID as STUDENT_3_2_ 
        from
            students studentx0_ 
        inner join
            notes studentnot2_ 
                on studentx0_.STUDENT_ID=studentnot2_.STUDENT_ID 
        inner join
            notes studentnot3_ 
                on studentx0_.STUDENT_ID=studentnot3_.STUDENT_ID 
        where
            studentx0_.STUDENT_ID=3;

这是我收到的错误消息:

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'as col_3_0_, studentnot3_.NOTE_ID as NOTE_ID1_2_, studentnot3_.NOTE_CONTENT as N' at line 1

现在我可以看到查询错误的地方，但它是由 Hibernate 生成的，而不是我可以控制的。我的查询字符串中是否需要更改某些内容才能获得我需要的结果。

我不想手动将结果映射到我的 DTO，有没有办法可以直接将 Student.java 中的 StudentNotes 映射到 StudentDTO.java 中的笔记

Answer 1

看起来这个查询是错误的。更好的方法是只找学生。您随时可以从学生那里获得笔记集。

Session session = getSessionFactory().openSession();
String queryString = from Student s where s.studentId = 3;
Query query = session.createQuery(queryString);
Student student = query.getSingleResult();
sysout(student.getNotes().size())

此外，我从未在 SELECT 子句中以这种方式检索集合；所以，不确定但你真的需要

join s.studentNotes

在您的查询中？不确定我的回答是否有帮助。