java - 可以在双向关系中创建两列吗？

Question

是否可以为双向关系创建一列？

我的实体:

@Entity
@Table(name = "subscription")
@Proxy(lazy = false)
public class Subscription {

@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
@Column(name = "subscription_id")
private long id;

@Column(name = "userid", nullable = false)
private String userId;

@Column(name = "saledate", nullable = false)
@Temporal(TemporalType.DATE)
private Date saleDate;

@Column(name = "finishdate", nullable = false)
@Temporal(TemporalType.DATE)
private Date finishDate;

@Column(name = "price", nullable = false)
private long price;

@Column(name = "description", nullable = false)
private String description;


@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, mappedBy = "subscription")
private List<VisitDate> visitDates = new ArrayList<>();
}



@Entity
@Table(name="visitdate")

public class VisitDate {
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
@Column(name = "id")
private long id;
private long subscription;

@Column(name = "date", nullable = false)
@Temporal(TemporalType.DATE)
private Date date;
@ManyToOne
@JoinColumn(name="subscription_id")
private Subscription associatedSub;
}

现在我在数据库中看到两列，有点困惑。

我不想保存相同的数据，而是想显示某天有多少用户访问的报告。

更新:

Answer 1

您不需要在 VisitDate 类中创建单独的字段“订阅”。 Hibernate 会自动创建一个字段来存储订阅 ID。代码需要稍微改动一下。

@Entity
@Table(name = "subscription")
public class Subscription {

  @Id
  @GeneratedValue(strategy= GenerationType.IDENTITY)
  @Column(name = "subscription_id")
  private long id;

  @Column(name = "userid", nullable = false)
  private String userId;

  @Column(name = "saledate", nullable = false)
  @Temporal(TemporalType.DATE)
  private Date saleDate;

  @Column(name = "finishdate", nullable = false)
  @Temporal(TemporalType.DATE)
  private Date finishDate;

  @Column(name = "price", nullable = false)
  private long price;

  @Column(name = "description", nullable = false)
  private String description;

  @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, mappedBy = "associatedSub")
  private List<VisitDate> visitDates = new ArrayList<>();
}

请注意，我已将 mappedBy 属性更改为指向上述类中的 associatedSub。

@Entity
@Table(name="visitdate")

public class VisitDate {
  @Id
  @GeneratedValue(strategy= GenerationType.IDENTITY)
  @Column(name = "id")
  private long id;

  @Column(name = "date", nullable = false)
  @Temporal(TemporalType.DATE)
  private Date         date;

  @ManyToOne
  @JoinColumn(name="subscription_id")
  private Subscription associatedSub;
}