@@ ManyToOne @OneToMany映射，外键为空

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>();
    }

如果表User和Offer中的映射正确，请提供帮助.user_id列为空值....：（

Answer 1

我不确定这些是否仅是实体的一部分，但为了使实体具有ID，您需要为其提供一个ID，并将相关字段注释为@Id。我也是@GeneratedValue（strategy = GenerationType.IDENTITY），所以每个表都将获得它自己的ID，而不是全局ID，否则，假设您添加了Offer，获得了带有值x的ID，然后添加了新的User，得到了带有值的ID x + 1等等...

@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>();
}`