根据 Hibernate documentation ，如果我们想使用 Map 作为实体之间的关联，可以使用多个注释。医生说:

Alternatively the map key is mapped to a dedicated column or columns. In order to customize the mapping use one of the following annotations:

@MapKeyColumn if the map key is a basic type. If you don't specify the column name, the name of the property followed by underscore followed by KEY is used (for example orders_KEY). @MapKeyEnumerated / @MapKeyTemporal if the map key type is respectively an enum or a Date. @MapKeyJoinColumn/@MapKeyJoinColumns if the map key type is another entity. @AttributeOverride/@AttributeOverrides when the map key is a embeddable object. Use key. as a prefix for your embeddable object property names. You can also use @MapKeyClass to define the type of the key if you don't use generics.

通过一些示例，我可以理解@MapKey 仅用于将键映射到目标实体的属性，并且该键仅用于获取记录。 @MapKeyColumn 用于将键映射到目标实体的属性，该键用于保存和获取记录。请告诉我这是否正确？

当我需要使用@MapKeyJoinColumn/@MapKeyJoinColumns & @MapKeyEnumerated/@MapKeyTemporal 时请告诉我

谢谢!

最佳答案

当您使用 Map 时，您始终需要关联至少两个实体。假设我们有一个与 Car 实体相关的 Owner 实体(Car 具有到 Owner 的 FK)。

因此，Owner 将拥有 Car(s) 的 Map:

Map<X, Car>

@MapKey

@MapKey 将为您提供Car's 属性，用于将Car 分组到其Owner。例如，如果我们在 Car 中有一个 vin(车辆识别号)属性，我们可以将其用作 carMap 键:

@Entity
public class Owner {
    @Id
    private long id;

    @OneToMany(mappedBy="owner")
    @MapKey(name = "vin")
    private Map<String, Car> carMap;
}

@Entity
public class Car {
    @Id
    private long id;

    @ManyToOne
    private Owner owner;

    private String vin;

}

@MapKeyEnumerated

@MapKeyEnumerated 将使用来自 Car 的 Enum，例如 WheelDrive:

@Entity
public class Owner {
    @Id
    private long id;

    @OneToMany(mappedBy="owner")
    @MapKeyEnumerated(EnumType.STRING)
    private Map<WheelDrive, Car> carMap;
}

@Entity
public class Car {
    @Id
    private long id;

    @ManyToOne
    private Owner owner;

    @Column(name = "wheelDrive")
    @Enumerated(EnumType.STRING)
    private WheelDrive wheelDrive;

}

public enum WheelDrive {
    2WD, 
    4WD;             
}

这将按车轮驱动类型对汽车进行分组。

@MapKeyTemporal

@MapKeyTemporal 将使用 Date/Calendar 字段进行分组，例如 createdOn。

@Entity
public class Owner {
    @Id
    private long id;

    @OneToMany(mappedBy="owner")
    @MapKeyTemporal(TemporalType.TIMESTAMP)
    private Map<Date, Car> carMap;
}

@Entity
public class Car {
    @Id
    private long id;

    @ManyToOne
    private Owner owner;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name="created_on")
    private Calendar createdOn;         
}

@MapKeyJoinColumn

@MapKeyJoinColumn 需要第三个实体，例如 Manufacturer 以便您拥有从 Owner 到 Car 的关联> 并且汽车也与 Manufacturer 关联，因此您可以按 Manufacturer 对所有 Owner's Cars 进行分组:

@Entity
public class Owner {
    @Id
    private long id;

    @OneToMany(mappedBy="owner")
    @MapKeyJoinColumn(name="manufacturer_id")
    private Map<Manufacturer, Car> carMap;
}

@Entity
public class Car {
    @Id
    private long id;

    @ManyToOne
    private Owner owner;

    @ManyToOne
    @JoinColumn(name = "manufacturer_id")
    private Manufacturer manufacturer;          
}

@Entity
public class Manufacturer {
    @Id
    private long id;

    private String name;
}

关于java - JPA和Hibernate中@MapKey、@MapKeyColumn和@MapKeyJoinColumn的区别，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/25439813/