JPA / Hibernate – Use a Map as an association

In this article we will see how to use a Map as an association between our entities, according to the Hibernate documentation, there are several annotations available (@MapKey, @MapKeyColumn, @MapKeyJoinColumn, @MapKeyEnumerated, @MapKeyTemporal, etc.) the documentation says:

Alternatively, the mapping key is mapped to one or more dedicated columns. To customize the mapping, use one of the following elements (annotations):

• @MapKeyColumn if the key is a base type. If you do not specify the column name, the property name followed by an underscore followed by KEY is used (for example KEY_ORDERS).
• @MapKeyEnumerated / @MapKeyTemporal if the key type is an enumeration or a date, respectively.
• @MapKeyJoinColumn / @MapKeyJoinColumns if the key type is another entity.
• @AttributeOverride / @AttributeOverrides when the mapping key is an embeddable object (@Embeddable).

Use the key. as a prefix for the property name of your embeddable object. You can also use @MapKeyClass to set the type of the key if you are not using generics.

By doing some examples, I am able to understand that @MapKey is just used to map the key to a property of the target entity and that key is used only to retrieve records. @MapKeyColumn is used to associate the key with a property of the target entity and this key is used to save and retrieve records.

When using a Map, you must always associate at least two entities. Suppose we have an entity Owner that relates to the entity Car (Car has an FK to Owner).

Thus, the owner will have a Map of car(s):

Map<X, Voiture>

@MapKey

The @MapKey will give you ownership of the car used to group a car to its owner. For example, if we have a level (Vehicle Identification Number) property in Car, we could use it as the mapCar key:

@Entity
public class Proprietaire {
    @Id
    private long id;
 
    @OneToMany(mappedBy="proprietaire")
    @MapKey(name = "niv")
    private Map<String, Voiture> mapVoiture;
}
 
@Entity
public class Voiture {
    @Id
    private long id;
 
    @ManyToOne
    private Proprietaire proprietaire;
 
    private String niv;
 
}

@MapKeyEnumerated

The @MapKeyEnumerated will use a Car Enum, such as Drive Wheels:

@Entity
public class Proprietaire {
    @Id
    private long id;
 
    @OneToMany(mappedBy="proprietaire")
    @MapKeyEnumerated(EnumType.STRING)
    private Map<RoueMotrice, Voiture > mapVoiture;
}
 
@Entity
public class Voiture {
    @Id
    private long id;
 
    @ManyToOne
    private Proprietaire proprietaire;
 
    @Column(name = "roue_motrice")
    @Enumerated(EnumType.STRING)
    private RoueMotrice roueMotrice;
 
}
 
public enum RoueMotrice {
    2RM, 
    4RM;             
}

This allows cars to be grouped according to their drive wheels.

@MapKeyTemporal

The @MapKeyTemporal will use a Date or Calendar field for grouping, like createdOn.

@Entity
public class Proprietaire {
    @Id
    private long id;
 
    @OneToMany(mappedBy="proprietaire")
    @MapKeyTemporal(TemporalType.TIMESTAMP)
    private Map<Date, Voiture> mapVoiture;
}
 
@Entity
public class Voiture {
    @Id
    private long id;
 
    @ManyToOne
    private Proprietaire proprietaire;
 
    @Temporal(TemporalType.TIMESTAMP)
    @Column(name="created_on")
    private Date createdOn;         
}

@MapKeyJoinColumn

The @MapKeyJoinColumn column requires a third entity, such as the manufacturer, in order to associate the owner with the car and the car with a manufacturer, which allows all of the owner's cars to be grouped by manufacturer:

@Entity
public class Proprietaire {
    @Id
    private long id;
 
    @OneToMany(mappedBy="proprietaire")
    @MapKeyJoinColumn(name="fabricant_id")
    private Map<Fabricant, Voiture> mapVoiture;
}
 
@Entity
public class Voiture {
    @Id
    private long id;
 
    @ManyToOne
    private Proprietaire proprietaire;
 
    @ManyToOne
    @JoinColumn(name = "fabricant_id")
    private Fabricant fabricant;          
}
 
@Entity
public class Fabricant {
    @Id
    private long id;
 
    private String nom;
}

I hope this article was useful to you. Thanks for reading it.

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