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

Question

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!

Answer 1

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> 

@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

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;              } 

This will group cars by their WheelDrive type.

@MapKeyTemporal

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

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; }