Get record from another table using JPA

Question

I have been trying to figure out how to do this for sometime without any luck and have not managed to find anything useful while search on Google either.

I have THREE tables:

HOTEL
 - id
 - name
 - local_id (foreign key)

DESCRIPTION
 - id
 - description
 - hotel_id (foreign key)
 - locale_id (foreign key)

LOCALE
 - id
 - local

I also have the following HOTEL DAO model:

@Entity
@Table(name = "HOTEL")
public class Hotel implements Serializable {

    @Column(name = "id")
private long id;

    @Column(name = "description")
    private HotelDescription description;
}

Using JPA, how can I retrieve the data from table DESCRIPTION based on hotel_id and locale_id to populate description in DAO model hotel?

Answer 1

Well, you also have HotelDescription JPA entity, right? So you can define bidirectional mapping for entities.

instead of

@Column(name = "description")
private HotelDescription description;

you should have something like

@OneToOne(mappedBy = "hotel", cascade = CascadeType.ALL)
private HotelDescription desc;

and on the other side, in HotelDescription you should have back mapping

@OneToOne
@JoinColumn(name = "hotel_id")
private Hotel hotel;

When you will extract Hotel entity, JPA will also fetch child entity (HotelDescription) for you.

if you want to use @OneToMany mapping it will be (many descriptions for one hotel)

@OneToMany(mappedBy = "hotel", cascade = CascadeType.ALL)
private HotelDescription desc;

and on the other side

@ManyToOne
@JoinColumn(name = "hotel_id")
private Hotel hotel;

In JPA you can use several types of mapping like OneToMany, ManyToMany... That's only basics. Find a tutorial. You may start here: http://docs.oracle.com/javaee/6/tutorial/doc/bnbqa.html (not the best one probably)

Oh. And make sure you annotate id with @Id

Answer 2

I would consider ditching the Locale table and working with java.util.Locale directly. Hibernate (not sure about other JPA implementations) has auto type conversion from char column to java.util.Locale. This would then look something like:

DESCRIPTION
 - id
 - description
 - hotel_id (foreign key)
 - locale

And Entity:

import java.util.Locale;

@Entity
    @Table(name = "HOTEL")
    public class Hotel implements Serializable {

        @Column(name = "id")
        private long id;

        @OneToMany
        @JoinColumn(name = "holiday_id", nullable = false)
        @MapKeyColumn(name = "locale_id")
        private Map<Locale, HotelDescription> descriptions;

        public String getDescriptionForLocale(Locale locale){

            //try an exact match e.g. en_us
            if(descriptions.containsKey(locale){
                return descriptions.get(locale).getDescription();
            }
            //try language only e.g. en
            else if (decsriptions.containsKey(locale.getLanguage){
                return descriptions.get(locale.getlanguage()).getDescription();
            }

            //return a default or null
            return ??
        }
    }