How to use a child association property as a Map key in JPA parent entity

Question

I'm having two entities Car and CarDescription where CarDescription is depending on another foreign key from the table Language.

What I' trying to accomplish is to have a HashMap in Car such that whenever I'm having a Car entity-object I am able to access all descriptions from the language id.

Entity Car.java

@Entity
@Table(name = "Car")
public class Car extends AbstractTimestampEntity implements Serializable {
    private static final long serialVersionUID = -5041816842632017838L;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name = "ID", unique = true, nullable = false)
    private Long id;

    @OneToMany(mappedBy="car")
    @MapKeyColumn(name = "language_ID")
    // @MapKey(name = "language") // does not work either 
    private Map<Long, CarDescription> carDescription = new HashMap<>(0);
}

Entity CarDescription.java

@Entity
@Table( name="car_description",
        uniqueConstraints = {
            @UniqueConstraint(columnNames={"language_id", "name"}) 
        }
)
public class CarDescription extends AbstractTimestampEntity implements Serializable {
    private static final long serialVersionUID = 2840651722666001938L;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name = "ID", unique = true, nullable = false)
    private Long id;

    @NotNull
    @ManyToOne
    private Car car;

    @NotNull
    @OneToOne
    private Language language;

    // ..
}

Entity Language.java

@Entity
public class Language implements Serializable {
    private static final long serialVersionUID = 3968717758435500381L;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="ID")
    private Long id;

    // ..
}

The problem I am having is that the mapping gives me a map from each CarDescription.id to CarDescription.

How can I accomplish a correct mapping?

@DraganBozanovic It's the same.. I'm always getting a map `CarDescription.id -> CarDescription` but what I want is `Language.id -> CarDescription`. I don't know but `@MapKey(name = "language")` does not work.. :/ — Stefan Falk, Jul 05 '15 at 22:33
It seems to be ignored. Are imports good, is it an annotation from `javax.persistence`? — Dragan Bozanovic, Jul 05 '15 at 22:46
@DraganBozanovic Yes, it's from `javax.persistence`. Another thing I've tried is `@MapKeyColumn (name = "language_ID")` but that didn't work either but in that case the map is just empty .. No idea what's not working here — Stefan Falk, Jul 05 '15 at 22:53
You tried with `@MapKeyColumn` together with `@MapKey` or without it? — Dragan Bozanovic, Jul 05 '15 at 22:58
@DraganBozanovic without it. Would I have to use both? I tried it due to the [documentation](https://docs.jboss.org/hibernate/orm/3.6/reference/en-US/html/collections.html#collections-indexed) that says: "*@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).*" — Stefan Falk, Jul 05 '15 at 22:58
@DraganBozanovic Hmm.. I just don't get it. It should work that way .. — Stefan Falk, Jul 05 '15 at 23:00
Your one-to-many association is wrong, it should be `mappedBy = "car"`. Then it should work with @MapKeyColumn. — Dragan Bozanovic, Jul 05 '15 at 23:10
@DraganBozanovic I've now tried `@OneToMany(cascade=CascadeType.ALL, mappedBy = "car") @MapKeyColumn (name = "language_ID")` but the map is here still empty :/ — Stefan Falk, Jul 05 '15 at 23:18

Vlad Mihalcea · Accepted Answer · 2015-07-06T07:43:15.813

4

In CarDescription you need to add the languageId property:

@Column(name = "language_id", insertable = false, updatable = false)
private Long languageId;

@NotNull
@OneToOne
@JoinColumn(name = "language_id")
private Language language;

public void setLanguage(Language language) {
    this.languageId = language.getId();
    this.language = language;
}

Then you can use it in the Car entity like this:

@OneToMany(mappedBy="car")
@MapKey(name = "languageId")
private Map<Long, CarDescription> carDescription = new HashMap<>(0);

edited Jul 06 '15 at 07:43

answered Jul 06 '15 at 05:32

Vlad Mihalcea

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Hi! Thanks for stopping by! :) Well, I've tried that but now I'm getting `org.hibernate.NonUniqueObjectException` "*a different object with the same identifier value was already associated with the session*" on `CarDescription`. Can't get around it :/ – Stefan Falk Jul 06 '15 at 06:57
That's something completely unrelated to the current question. That can happen when you call `Session.saveOrUpdate(entity)` when the entity is already associated to the current Session. You need to review your persistence logic thoroughly. – Vlad Mihalcea Jul 06 '15 at 07:00
It seems that the problem does come from `@MapsId`. The creation statement is now giving me `primary key (language_ID)` which won't work. Is there no other way to create this mapping? – Stefan Falk Jul 06 '15 at 07:41
Right, that only works for identifiers. I changed the answer to using `insertable = false, updatable = false`. Check the updated version. – Vlad Mihalcea Jul 06 '15 at 07:43
Awesome! Thank you very much I don't know if I would have solved that ever by myself! Hibernate has a ridiculously flat learning curve.. ^^ – Stefan Falk Jul 06 '15 at 07:58
Great trick! However, why did it not work with `@MapKeyColumn`? – Dragan Bozanovic Jul 06 '15 at 10:30
It should have worked with `@MapKeyColumn` as you suggested, so it must have failed because of some Hibernate bug. Could you fill a JIRA issue? – Vlad Mihalcea Jul 06 '15 at 10:40
Yes, I will try to reproduce it in a test case and create a jira issue. Thanks! – Dragan Bozanovic Jul 06 '15 at 10:57
Shouldn't `@MapKeyJoinColumn ` be used? See https://stackoverflow.com/questions/25439813/difference-between-mapkey-mapkeycolumn-and-mapkeyjoincolumn-in-jpa-and-hiber#25447741 – bgraves Mar 18 '19 at 13:21

How to use a child association property as a Map key in JPA parent entity

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