JPA和Hibernate中@MapKey、@MapKeyColumn和@MapKeyJoinColumn的区别

问题描述

根据 Hibernate 文档，有多个注释如果我们想使用 Map 作为我们实体之间的关联，则可用.医生说:

As per Hibernate documentation, there are multiple annotations available if we want to use Map as an association between our entities. The doc says:

或者，映射键被映射到一个或多个专用列.为了自定义映射使用以下之一注释:

@MapKeyColumn 如果映射键是基本类型.如果您不指定列名，属性名后跟下划线使用 by KEY(例如 orders_KEY).@MapKeyEnumerated/@MapKeyTemporal 如果映射键类型分别是枚举或日期.@MapKeyJoinColumn/@MapKeyJoinColumns 如果地图键类型是另一个实体.@AttributeOverride/@AttributeOverrides 当映射键是一个可嵌入对象.使用钥匙.作为可嵌入对象的前缀属性名称.您还可以使用@MapKeyClass 来定义类型如果你不使用泛型，关键是.

@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.

通过一些例子，我能够理解@MapKey 只是用于将键映射到目标实体的属性，而此键仅用于获取记录.@MapKeyColumn 用于将键映射到目标实体的属性，此键用于保存和获取记录.请告诉我这是否正确?

By doing some examples I am able to understand that @MapKey is just used to map the key to a property of target entity and this key is used only for fetching records. @MapKeyColumn is used to map the key to a property of target entity and this key is used to save as well as fetching records. Please let me know if this is correct?

另外请让我知道何时需要使用@MapKeyJoinColumn/@MapKeyJoinColumns &@MapKeyEnumerated/@MapKeyTemporal

Also please let me know when I need to use @MapKeyJoinColumn/@MapKeyJoinColumns & @MapKeyEnumerated / @MapKeyTemporal

谢谢！

推荐答案

当您使用 Map 时，您总是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 有一个到 Owner 的 FK).

When you use a Map you always need to associate at least two entities. Let's say we have an Owner entity that relates to the Car entity (Car has a FK to Owner).

因此，Owner 将拥有 Car(s) 的 Map:

So, the Owner will have a Map of Car(s):

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，我们可以将它用作 carMap 键:

@MapKey

The @MapKey will give you the Car's property used to group a Car to its Owner. For instance, if we have a vin (Vehicle Identification Number) property in Car, we could use it as the carMap key:

@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 的枚举，例如 WheelDrive:

@MapKeyEnumerated

The @MapKeyEnumerated will use an Enum from Car, like 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.

This will group cars by their WheelDrive type.

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

The @MapKeyTemporal will use a Date/Calendar field for grouping, like 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 car 还与 Manufacturer 关联，因此您可以按 Manufacturer 将所有 Owner's Cars 分组:

@MapKeyJoinColumn

The @MapKeyJoinColumn requires a third entity, like Manufacturer so that you have an association from Owner to Car and car has also an association to a Manufacturer, so that you can group all Owner's Cars by Manufacturer:

@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;
}

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