@MapKeyJoinColumn 在映射表中显示为空

Question

我是休眠新手，目前正在尝试使用实体类型为键和值进行映射。我在映射表中保留密钥（country_id）时遇到问题，因为它在列名下一直表示为null：mapkey_country_id，即使我明确使用@MapKeyJoinColumn 将列名指定为@987654325 @。

有关问题的更多上下文：

这是一个基本问题，我有一个带有Map<Country, Capital> 属性的Continent 实体。所以我写了以下代码：

大陆实体

@Entity
public class Continent {
        
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long cont_id;
    
    private String continent;

    @OneToMany(mappedBy="cname")
    @MapKeyJoinColumn(name ="country_id",nullable=false)
    private Map<Country, Capital> countries;

    //   followed by setters and getters...
}

资本实体

@Entity
public class Capital {

    private String capital;

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

    @ManyToOne
    @JoinTable(name="cont_cap",
        joinColumns= @JoinColumn(name="capital_id"),
        inverseJoinColumns =@JoinColumn(name="cont_id")
    )
    private Continent cname;

    @OneToOne
    @JoinColumn(name="country_id")
    private Country country;
    
    // followed by setters and getters...
}

国家实体

@Entity
public class Country {

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

    private String country;

    //  followed by setters and getters...
}

主要方法

public static void main(String[] args) {
    SessionFactory sesfac = new AnnotationConfiguration().configure().buildSessionFactory();
    
    Session session= sesfac.openSession();
    
    Transaction tx = session.beginTransaction();
    //country2
    Country country2= new Country();
    country2.setCountry("USA");
    
    Capital cap2= new Capital();
    cap2.setCapital("Washington, D.C.");
    cap2.setCountry(country2);
    
    Continent cont2= new Continent();
    cont2.setCname("North America");
    HashMap<Country, Capital> hm2 = new HashMap<Country, Capital>();
    hm2.put(country2, cap2);
    cont2.setCountries(hm2);
    cap2.setCname(cont2);
            
    session.save(country2);
    session.save(cap2);
    session.save(cont2);
    tx.commit();
    session.close();
}

Answer 1

我猜你应该在这种情况下使用@MapKey 注释而不是@MapKeyJoinColumn：

@Entity
public class Continent {
    
    @OneToMany(mappedBy="cname", cascade = {CascadeType.PERSIST})
    @MapKey(name ="country")
    private Map<Country, Capital> countries;

    // ...
}

正如documentation 中解释的那样：

@MapKey 注释用于定义实体属性，用作相关java.util.Map 的键。

请注意，我将cascade = {CascadeType.PERSIST} 添加到@OneToMany。您还可以通过以下方式更正您的 Capital 实体：

@Entity
public class Capital {

    @OneToOne(cascade = {CascadeType.PERSIST})
    @JoinColumn(name="country_id")
    private Country country;
    
    // followed by setters and getters...
}

然后持久化操作将如下所示：

Country country = new Country();
country.setCountry("USA");
    
Capital capital = new Capital();
capital.setCapital("Washington, D.C.");
capital.setCountry(country);
    
Continent continent = new Continent();
continent.setCname("North America");
 
HashMap<Country, Capital> countries = new HashMap<Country, Capital>();
countries.put(country, capital);
continent.setCountries(countries);
capital.setCname(continent);

// It will save also Country and Capital due to the CascadeType.PERSIST usage
session.persist(continent);