@ManyToOne @OneToMany 映射，外键为 null

Question

@Entity
@Data
@NoArgsConstructor
public class Offer {
    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name = "user_id", referencedColumnName = "id")
    private User user;
}

@Data
@EqualsAndHashCode
@Entity
@NoArgsConstructor
public class User  {
    @OneToMany(mappedBy = "user",cascade = CascadeType.ALL,fetch=FetchType.LAZY)
    private Set<Offer> offers = new HashSet<Offer>();
}

Please help if the mapping is correct in table User and Offer.user_id column have null values....:(

Answer 1

I'm not sure if these are only parts of the entities but in order for the entity to have an id, you need to provide it with one and annotate the relevant field as @Id. I also use @GeneratedValue(strategy = GenerationType.IDENTITY) so each table will get it's own id (generated by Hibernate, you don't provide the id when you save a new entity and not a global id, otherwise let's say you add an Offer, you get id with value x, then add new User you get id with value x+1 and so on...

@Entity
@Data
@NoArgsConstructor
public class Offer {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name = "user_id", referencedColumnName = "id")
    private User user;

}



@Data
@EqualsAndHashCode
@Entity
@NoArgsConstructor
public class User  {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
   private long id;
     @OneToMany(mappedBy = "user",cascade = CascadeType.ALL,fetch=FetchType.LAZY)
    private Set<Offer> offers = new HashSet<Offer>();
}`