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

Difference between @MapKey, @MapKeyColumn and @MapKeyJoinColumn in JPA and Hibernate

提问by Chaitanya

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

根据Hibernate 文档，如果我们想使用 Map 作为实体之间的关联，则有多个注释可用。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.

或者，映射键被映射到一个或多个专用列。为了自定义映射，请使用以下注释之一：

@MapKeyColumn 如果映射键是基本类型。如果不指定列名，则使用属性名称后跟下划线后跟 KEY（例如 orders_KEY）。@MapKeyEnumerated / @MapKeyTemporal 如果映射键类型分别是枚举或日期。@MapKeyJoinColumn/@MapKeyJoinColumns 如果映射键类型是另一个实体。@AttributeOverride/@AttributeOverrides 当映射键是可嵌入对象时。使用钥匙。作为可嵌入对象属性名称的前缀。如果不使用泛型，也可以使用 @MapKeyClass 来定义键的类型。

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?

通过做一些例子，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。请让我知道这是否正确？

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

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

Thanks!

谢谢！

采纳答案by Vlad Mihalcea

When you use a Mapyou always need to associate at least two entities. Let's say we have an Ownerentity that relates to the Carentity (Carhas a FK to Owner).

当您使用 a 时，Map您总是需要关联至少两个实体。假设我们有一个与Owner实体相关的Car实体（Car有一个 FK to Owner）。

So, the Ownerwill have a Mapof Car(s):

所以，Owner就有了Map的Car(s)：

Map<X, Car>

@MapKey

@MapKey

The @MapKeywill give you the Car'sproperty used to group a Carto its Owner. For instance, if we have a vin(Vehicle Identification Number) property in Car, we could use it as the carMapkey:

该@MapKey会给你Car's用来集团的物业Car其Owner。例如，如果我们在 中有一个vin(Vehicle Identification Number) 属性Car，我们可以将它用作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

The @MapKeyEnumeratedwill use an Enum from Car, like WheelDrive:

该@MapKeyEnumerated会使用枚举从Car，像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;             
}

This will group cars by their WheelDrive type.

这将按其 WheelDrive 类型对汽车进行分组。

@MapKeyTemporal

@MapKeyTemporal

The @MapKeyTemporalwill use a Date/Calendarfield for grouping, like createdOn.

该@MapKeyTemporal会使用Date/Calendar场进行分组，像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

The @MapKeyJoinColumnrequires a third entity, like Manufacturerso that you have an association from Ownerto Carand car has also an association to a Manufacturer, so that you can group all Owner'sCarsby Manufacturer:

将@MapKeyJoinColumn需要第三个实体，就像Manufacturer让你从这样一个联合体Owner，以Car与汽车也已经关联到Manufacturer，让您可以将所有Owner'sCars的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;
}

回答by Ryan

Here's a working example of using @MapKey with @OneToMany with a composite @IdClass. It's obviously not the only way to accomplish the objective here, but I felt this was the most maintainable.

这是将@MapKey 与@OneToMany 与复合@IdClass 一起使用的工作示例。这显然不是实现目标的唯一方法，但我觉得这是最容易维护的。

@Entity
@Table(name = "template_categories")
@IdClass(TemplateCategoryId.class)
public class TemplateCategory implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    long orgId;
    @Id
    long templateId;

    @OneToMany(targetEntity = TemplateEntry.class)
    @JoinColumns( {
        @JoinColumn(name = "orgId",  referencedColumnName = "orgId"),
        @JoinColumn(name = "templateId",  referencedColumnName = "templateId")
        }
    )
    @MapKey(name="key")
    private Map<String, TemplateEntry> keyMap;

source code: https://github.com/in-the-keyhole/jpa-entity-map-examples/blob/master/src/main/java/com/example/demo/mapkey/entity/TemplateCategory.java

源代码：https: //github.com/in-the-keyhole/jpa-entity-map-examples/blob/master/src/main/java/com/example/demo/mapkey/entity/TemplateCategory.java