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;
}