如何在JPA中建立一对多关系

Question

I'm not sure what I'm missing to make a onetomany relationship using JPA.

Field username_id in second table is created as MUL key, I'm expecting Foreign Key, what's wrong?

@Entity
@Table(name="userandpassword")
public class LoginAndPassword {

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

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

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

    @OneToMany(cascade = CascadeType.ALL)
    List<GradeEntity> g;

//getters, setters

@Entity
@Table(name = "grades")
public class GradeEntity {

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

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

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

    @ManyToOne
    @JoinColumn(name= "username_id")
    private LoginAndPassword loginAndPassword;

//getters,setters

Answer 1

You need to add a mappedBy in the @OneToMany

 @OneToMany(cascade = CascadeType.ALL, mappedBy = "loginAndPassword")
 List<GradeEntity> g;

Answer 2

Basically you need to specify the

@OneToMany(mappedBy = "loginAndPassword", cascade = CascadeType.ALL)
List<GradeEntity> g;

This is a good tutorial explaining the JPA and Hibernate OneToMany relationship mapping, I highly recommend you to check it out.

JPA and Hibernate One To Many Relationship Mapping Example with Spring Boot and MySQL