JPA和Hibernate中的@MapKey，@MapKeyColumn和@MapKeyJoinColumn之间的区别 [英] Difference between @MapKey, @MapKeyColumn and @MapKeyJoinColumn in JPA and Hibernate

问题描述

根据 Hibernate文档，有多个注释如果我们想使用Map作为我们的实体之间的关联可用。该文档说：

或者，映射键映射到专用的一列或多列。
为了自定义映射，使用以下
注释之一：

$ b @MapKeyColumn如果map键是基本类型。如果您未指定
列名称，则使用该属性的名称，后跟紧跟着
（通过KEY）的下划线（例如orders_KEY）。 @MapKeyEnumerated /
@MapKeyTemporal如果地图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map键是
可嵌入对象时。使用键。作为可嵌入对象
属性名称的前缀。你也可以使用@MapKeyClass来定义
的类型，如果你不使用泛型。

通过做一些例子我能够理解@MapKey只是用来将键映射到目标实体的属性，并且这个键仅用于获取记录。 @MapKeyColumn用于将键映射到目标实体的属性，该键用于保存和提取记录。请让我知道如果这是正确的？

另外，请让我知道，当我需要使用@ MapKeyJoinColumn / @ MapKeyJoinColumns& @MapKeyEnumerated / @MapKeyTemporal

感谢！

解决方案

一个 Map 你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FK到所有者）。 因此，所有者将有一个 Map Car（s）：

 地图< X，Car> 
  

1. @MapKey 将为您提供 Car's 字段，用于将多个 Car（s）分组到 $ <$ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ Car ，例如 WheelDrive ：



     @Entity 
   公共类所有者{
    @Id 
   私人长ID; 
    
    @OneToMany（mappedBy =owner）
    @MapKeyEnumerated（EnumType.STRING）
   私人地图< WheelDrive，Car> carMap; 
   } 
    
    @实体
   公共类汽车{
    @Id 
   私人长ID; 
    
    @ManyToOne 
   私人所有者; 
    
    @Column（name =wheelDrive）
    @Enumerated（EnumType.STRING）
   私人WheelDrive wheelDrive; 
    
   } 
    
    public enum WheelDrive {
    2WD，
    4WD; 
    
     

   这会按照他们的WheelDrive类型对汽车进行分类。




2. @MapKeyTemporal 将使用日期 / 日历分组字段，例如 createdOn

     @实体
   公共类所有者{
    @Id 
   私人长ID; 
    
    @OneToMany（mappedBy =owner）
    @MapKeyTemporal（TemporalType.TIMESTAMP）
    private Map< Date，Car> carMap; 
   } 
    
    @实体
   公共类汽车{
    @Id 
   私人长ID; 
    
    @ManyToOne 
   私人所有者; 
    
    @Temporal（TemporalType.TIMESTAMP）
    @Column（name =created_on）
   私人日历createdOn; 
    
     




3. @MapKeyJoinColumn 需要第三个实体，例如制造商，以便您拥有从所有者到的关联。 Car 和car也与制造商有关联，以便您可以将所有所有者 Cars by 制造商：



     @Entity 
   公共类所有者{
    @Id 
   私人长ID; 
    
    @OneToMany（mappedBy =owner）
    @MapKeyJoinColumn（name =manufacturer_id）
    private Map< Manufacturer，Car> carMap; 
   } 
    
    @实体
   公共类汽车{
    @Id 
   私人长ID; 
    
    @ManyToOne 
   私人所有者; 
    
    @ManyToOne 
    @JoinColumn（name =manufacturer_id）
   私人制造商制造商; 
   } 
    
    @实体
   公共类制造商{
    @Id 
   私人长ID; 
    
   私人字符串名称; 
   } 
     







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

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.

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?

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

Thanks!

解决方案

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).

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

Map<X, Car>

1. The @MapKey will give you the Car's field used to group multiple Car(s) to an Owner:

2. @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;             
   }   
   

   This will group cars by their WheelDrive type.

3. @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;         
   }   
   

4. @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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