嘗試在Hibernate中添加主復合鍵時，“無法添加或更新子行：外鍵約束失敗”

Question

我正在從事學校成績簿實施的項目，其中包含以下實體：學生，科目和成績。 每個年級都有一個科目，一個學生，日期和值。 我正在嘗試實現這種關系，因此我決定一個年級要有一個由主題ID，學生ID和日期組成的復合主鍵，這是我的實體類

學生：

@Entity
@Table(name="Students")
public class Student {
     @Id @Column(name="Student_Id")
     private int studentId;
     @Column(name="First_Name")
     private String firstName;
     @Column(name="Last_Name")
     private String lastName;
     //getters and setters
}

學科：

@Entity
@Table(name="Subjects")
public class Subject {
    @Id @Column(name="Subject_Id")
    private int subjectId;
    @Column(name="Subject_Name")
    private String subjectName;
    //getters and setters
}

等級（在這里，我通過在Student_Id ， Subject_Id和Date上添加@Id標記來創建復合主鍵，其中Student_Id和Subject_Id也是外鍵）：

@Entity
@Table(name="Grades")
public class Grade implements Serializable{
    @Id
    @ManyToOne @JoinColumn(name="Student_Id", foreignKey=@ForeignKey(name="FK_Grades_Student"))
    @Cascade(CascadeType.SAVE_UPDATE)
    private Student student;
    @Id
    @ManyToOne @JoinColumn(name="Subject_Id", foreignKey=@ForeignKey(name="FK_Grades_Subject"))
    @Cascade(CascadeType.SAVE_UPDATE)
    private Subject subject;
    @Column(name="Date")
    @Id
    private Date date;
    @Column(name="Letter_Value") //A,B,C,D,F
    private String letterValue;
    //getters and setters
}

這是我從Main運行它的方式：

public class Main {

    public static void main(String[] args) {
        Subject subject = new Subject(); subject.setSubjectId(1); subject.setSubjectName("Biology");
        Student student = new Student(); student.setStudentId(1); student.setFirstName("Jack"); student.setLastName("Smith");
        Grade grade = new Grade(); grade.setStudent(student); grade.setSubject(subject); grade.setDate(new Date()); grade.setLetterValue("B"); 

        SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
        Session session = sessionFactory.openSession();
        session.beginTransaction();
        session.save(grade);
        session.getTransaction().commit();
        session.close();
        sessionFactory.close();
    }
}

運行后，出現以下異常：

com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: 

Cannot add or update a child row: a foreign key constraint fails (`grades_book_db`.`grades`, CONSTRAINT `FK_Grades_Subject` FOREIGN KEY (`Subject_Id`) REFERENCES `subjects` (`Subject_Id`))
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:425)
    at com.mysql.jdbc.Util.getInstance(Util.java:408)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:935)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3970)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3906)
    at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2524)
    at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2677)
    at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2549)
    at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1861)
    at com.mysql.jdbc.PreparedStatement.executeUpdateInternal(PreparedStatement.java:2073)
    at com.mysql.jdbc.PreparedStatement.executeUpdateInternal(PreparedStatement.java:2009)
    at com.mysql.jdbc.PreparedStatement.executeLargeUpdate(PreparedStatement.java:5098)
    at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1994)
    at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:204)
    ... 18 more

如何解決此異常，如何實現上面提到的組合鍵？ 我正在使用MySQL DBMS。

Answer 1

嘗試先持久保存Student和Subject對象。 如果您不持久化，Hibernate將無法在數據庫中找到這些對象。