JPA - 实体设计问题 [英] JPA - Entity design problem

问题描述

我正在开发Java桌面应用程序并使用JPA进行持久化。我有一个问题如下：

I am developing a Java Desktop Application and using JPA for persistence. I have a problem mentioned below:

我有两个实体：

• 国家


• 城市

国家/地区具有以下属性：

Country has the following attribute:

• CountryName（PK）

City具有以下属性：

City has the following attribute:

• CityName

现在可以有两个城市在两个不同的国家/地区，同名，数据库中的City表的primaryKey是一个复合主键，由 CityName 和 CountryName 。

Now as there can be two cities with same name in two different countries, the primaryKey for City table in the datbase is a composite primary key composed of CityName and CountryName.

现在我的问题是如何实现 City的主键作为Java中的实体

   @Entity
   public class Country implements Serializable {
       private String countryName;

       @Id
       public String getCountryName() {
           return this.countryName;
       }
   }

  @Entity
  public class City implements Serializable {
           private CityPK cityPK;
           private Country country;

           @EmbeddedId
           public CityPK getCityPK() {
               return this.cityPK;
           }
   }


   @Embeddable
   public class CityPK implements Serializable {
       public String cityName;
       public String countryName;
   }

现在我们知道来自 Country <的关系/ code>到城市是 OneToMany 并在上面的代码中显示这种关系，我添加了一个 country City 类中的变量。

Now as we know that the relationship from Country to City is OneToMany and to show this relationship in the above code, I have added a country variable in City class.

但是接下来我们将重复数据（ countryName ）存储在 City class'对象中的两个位置：<$ c $中的一个c> country 对象和 cityPK 对象中的其他对象。

But then we have duplicate data(countryName) stored in two places in the City class' object: one in the country object and other in the cityPK object.

但另一方面手，都是必要的：

But on the other hand, both are necessary:

• countryName in cityPK 对象是必要的，因为我们以这种方式实现复合主键。

• countryName in cityPK object is necessary because we implement composite primary keys in this way.

countryName in country 对象是必要的，因为它是显示对象之间关系的标准方式。

countryName in country object is necessary because it is the standard way of showing relashionship between objects.

如何解决这个问题？

推荐答案

countryName in CityPK 应使用 @Column标记为只读（insertable = false，updatable = false ）并且 countryName s应该映射到同一列（使用 name 属性） ：

countryName in CityPK should be marked read-only using @Column(insertable = false, updatable = false) and both countryNames should be mapped to the same column (using name property):

  @Entity
  public class City implements Serializable {
           @EmbeddedId
           private CityPK cityPK;

           @ManyToOne
           @JoinColumn(name = "countryName")
           private Country country;
  }


   @Embeddable
   public class CityPK implements Serializable {
       public String cityName;

       @Column(name = "countryName", insertable = false, updatable = false)
       public String countryName;
   }

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