Creating entity for database

Question

For example, I have database with table book (id, title, themeId, authorId), table author (id, name) and table theme (id, name). I dont know, should i create entity exactly like in database (and then i will have problems with join queries):

class Book {
    private id;
    String title;
    int bookThemeId;
    int bookAuthorId;
}

or create something like this:

class Book {
    private id;
    String title;
    BookTheme bookTheme;
    Author bookAuthor;
}

in second case i will have problems if authorId or themeId is null (it may be) and problems with getting from DAO in general, if i want get only books list (problems - fields is null, and very bulky builder for Book entity).

Second option would preclude books with multiple themes and authors. — P.Salmon
– P.Salmon, Commented Apr 3, 2020 at 7:37
@P.Salmon its not a big deal for this moment, its just example — ranger gref
– ranger gref, Commented Apr 3, 2020 at 7:49

Patrick Santana · Accepted Answer · 2020-04-03 07:36:12Z

2

I would go with the second option because is more object oriented. If you go with the first option, you will need to do a second query to get the details.

@Entity
@Table(name = "books")
public class Book {

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

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "theme_id", referencedColumnName = "id")
    private BookTheme bookTheme;

    // ... getters and setters
}

Your id will be null when fields are not created, right? For instance, when it is saved on database, it will always have an id.

answered Apr 3, 2020 at 7:36

Patrick Santana

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3 Comments

ranger gref Over a year ago

Thank you for answer, yes i like second option too, but i dont use annotation and JPA for the time being learn servlets so i dont understand what exactly you mean by using annotations. About ids - i mean foreign id, like themeId, it may be null, so we can catch NullPointerException while using entities from second option.

Patrick Santana Over a year ago

I would not catch NullPointException instead just check if a variable if null or not. It is cleaner.

ranger gref Over a year ago

Ok, i think null-checking low price for easy entity using

Birju B · Accepted Answer · 2020-04-03 07:50:22Z

1

I think you should go with 2nd approach. There will be no issue if your BookTheme r Author will be null if you use optional=true. For bulky builder, you can use fetchtype as lazy.

@Entity
@Table(name = "Book")
public class Book {

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

    @OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY, optional=true)
    @JoinColumn(name = "theme_id", referencedColumnName = "id")
    private BookTheme bookTheme;

    // ... getters and setters
}

Hope, This will work!

answered Apr 3, 2020 at 7:50

Birju B

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4 Comments

ranger gref Over a year ago

Thank you for answer, i get you, but i dont use annotations and JPA.

Birju B Over a year ago

If you are not using that thn you should go with 1st approach

ranger gref Over a year ago

Why should i go with 1st option, if i dont use annotation?

Birju B Over a year ago

If you go with 2nd option thn it will be bulky. In first option if you use primitive thn u can handle null thing and also get only id rather thn whole object.

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