CODEWITHSUNDEEP
javahibernateormone-to-many
Grook
Grook

Reputation: 405

Hibernate OneToMany. Getting empty list when loadin from DB

I'm new to Hibernate so my question can be silly a bit, by i'm stucked and will be glad to get help.

I have two entities: Book and Tag with following structure:

@Entity
public class BookEntity{

@Column(nullable = false)
private String title;
@Column(nullable = false)
private String author;
private String publisher;
private int edition;
private int yearOfPublishing;

@Id
@Column(name = "isbn")
private String isbn;

@ElementCollection(fetch = FetchType.EAGER)
@OneToMany(cascade = CascadeType.ALL,fetch = FetchType.EAGER)
@JoinTable(joinColumns = { @JoinColumn(name = "isbn") },
           inverseJoinColumns = { @JoinColumn(name = "tagId") })
private List<Tag> tags;
//getters & setters

@Entity
public class Tag implements Serializable {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private int tagId;
private String tagValue;
//getters & setters

Insertion works fine, here is the HQL queries:

insert into PUBLIC.BookEntity 
(author, edition, publisher, title, yearOfPublishing, isbn) 
values (?, ?, ?, ?, ?, ?)

insert into PUBLIC.Tag
(tagId, tagValue) 
values (null, ?)

Selection queries looks fine also:

select
        bookentity0_.isbn as isbn35_1_,
        bookentity0_.author as author35_1_,
        bookentity0_.edition as edition35_1_,
        bookentity0_.publisher as publisher35_1_,
        bookentity0_.title as title35_1_,
        bookentity0_.yearOfPublishing as yearOfPu6_35_1_,
        tags1_.isbn as isbn35_3_,
        tag2_.tagId as tagId3_,
        tag2_.tagId as tagId36_0_,
        tag2_.tagValue as tagValue36_0_ 
    from
        PUBLIC.BookEntity bookentity0_ 
    left outer join
        PUBLIC.BookEntity_Tag tags1_ 
            on bookentity0_.isbn=tags1_.isbn 
    left outer join
        PUBLIC.Tag tag2_ 
            on tags1_.tagId=tag2_.tagId 
    where
        bookentity0_.isbn=?

But when i'm loading BookEntity from database i'm getting correct object with empty list of Tags. The loading object from database :

public T read(PK id) {
    LOG.debug("Reading by id={}", id.toString());
    return (T)getSession().get(type, id);
}

where T is BookEntity, type is Class and PK is String.

What am i doing wrong? Thanks in advance.

Upvotes: 0

Views: 6513

Answers (1)

dcernahoschi
dcernahoschi

Reputation: 15250

First of all choosing isbn as a primary key is not the most inspired idea. What if the user makes a typo and enters a wrong isbn?

Secondly, it seems to me that you are trying to map a many to many relation from books to tags. Or maybe a one to many?

For many to many use:

@ManyToMany
@JoinTable(name = "book_tag",
   joinColumns = {@JoinColumn(name = "isbn")},
   inverseJoinColumns = {@JoinColumn(name = "tag_id")}
)

For one to many use:

@OneToMany(cascade = CascadeType.ALL,fetch = FetchType.EAGER)
@JoinColumn(name = "isbn", nullable = false)

But you better replace isbn with a book_id.

Upvotes: 1

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