休眠，无法保存多对一关系

Question

I've been studying tutorials of @ManyToOne hibernate annotation, but don't fully understand something. To explain simply, here is piece of code:

@Entity
@Table
public class Student {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private long id;
@Column
private String name;
@OneToMany(mappedBy = "student", fetch = FetchType.LAZY)
private Set<Book> bookList;
// getters, setters...
}

and

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

@Id
@Column(name = "ID")
@GeneratedValue(strategy = GenerationType.IDENTITY)
private long id;
@Column(name = "NAME", unique = true, nullable = false, length = 100)
private String name;
@Column(name = "DESCRIPTION", unique = true, nullable = false, length = 100)
private String description;
@ManyToOne(fetch = FetchType.LAZY, optional = true)
@JoinTable(name = "CATALOG", joinColumns = @JoinColumn(name = "ID_BOOK"),  inverseJoinColumns = @JoinColumn(name = "ID_STUDENT"))
private Student student;
// getters, setters...
}

and, when on those two classes I execute:

Session session = factory.getCurrentSession();
    //start
    System.out.println("start!");
    Book book = new Book();
    book.setName("bulk");
    book.setDescription("hulk");
    //book set
    session.beginTransaction();
    session.save(book);
    //book saved
    Student stud = new Student();
    stud.setName("Mark");
    //stud set
    Set<Book> books = new HashSet<>();
    book.setStudent(stud);
    books.add(book);
    //book add to list
    stud.setBookList(books);
    //list added to stud
    session.save(stud);

    session.getTransaction().commit();

the result is obviously:

start!
Hibernate: insert into book (DESCRIPTION, NAME) values (?, ?)
Hibernate: insert into Student (name) values (?)
Hibernate: insert into CATALOG (ID_STUDENT, ID_BOOK) values (?, ?)
end

Now when I do the same (in my opinion, obviously, else it would work) with these two classes:

@Entity
@Table(name = "client",
    uniqueConstraints = {
        @UniqueConstraint(columnNames = {"client_id"})})
public class Client {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "client_id", nullable = false, unique = true, length = 11)
private int id;
@Column(name = "client_name", length = 60, nullable = true)
private String name;
@Column(name = "client_comment", length = 60, nullable = true)
private String comment;
@Column(name = "client_activated", nullable = true)
private boolean activated;

@OneToMany(mappedBy = "client", fetch = FetchType.LAZY)
Set<AdditionalProperty> propertiesList;

// start of class properties

@Entity
@Table(name = "user_properties",
    uniqueConstraints = {
        @UniqueConstraint(columnNames = {"property_id"})})
public class AdditionalProperty {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "property_id")
private int id;
@Column(name = "property_name", length = 60, nullable = true)
private String name;
@Column(name = "property_type", length = 60, nullable = true)
private String propertyType;
//@ManyToOne(cascade = CascadeType.ALL)
//@JoinColumn(name = "property_to_client")
//@ManyToOne(fetch = FetchType.LAZY,optional=true)
//@JoinTable(name = "ref_client_to_property", joinColumns = @JoinColumn(name = "ref_property_id"), inverseJoinColumns = @JoinColumn(name = "ref_client_id"))
@ManyToOne(fetch = FetchType.LAZY, optional = true)
@JoinTable(name = "ref_client_to_prop", joinColumns = @JoinColumn(name = "id_prop"), inverseJoinColumns = @JoinColumn(name = "id_client"))
private Client client;

when I execute this code:

AdditionalProperty prop = new AdditionalProperty();
    prop.setName("testf2");
    prop.setPropertyType("testt2");
    AdditionalProperty prop2 = new AdditionalProperty();
    prop2.setName("wat2");
    prop2.setPropertyType("wat");
    //props set
    new HibernateDAOAdditionalProperty().create(prop);
    new HibernateDAOAdditionalProperty().create(prop2);
    System.out.println("prop set");
    //props saved


    Client client = new Client();
    client.setName("cascadeable");
    client.setComment("ho-ho-ho");
    client.setActivated(false);
    //cli set
    Set<AdditionalProperty> propList = new HashSet<>();
    prop.setClient(client);
    prop2.setClient(client);
    propList.add(prop);
    propList.add(prop2);
    // props added to list
    client.setPropertiesList(propList);
    // list added to client
    HibernateDAOClient dao = new HibernateDAOClient();
    dao.create(client);

the result in query is not what I expect:

Hibernate: insert into user_properties (property_name, property_type) values (?, ?)
Hibernate: insert into user_properties (property_name, property_type) values (?, ?)
prop set
Hibernate: insert into client (client_activated, client_comment, client_name) values (?, ?, ?)

Where I expect:

Hibernate: insert into ref_client_to_prop (id_client, id_prop) values (?, ?)

Please help, what am I doing wrong? I need to save their relationship in database, but somehow I can't.

PS to add to confusion:

when I use this class:

public class ClientProperty {
@Id
@Column(name = "ID")
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;
@ManyToOne(cascade=CascadeType.ALL)
@JoinColumn(name="id_client")
private Client client;
@ManyToOne(cascade=CascadeType.ALL)
@JoinColumn(name="id_characteristic")
private AdditionalProperty property;
@Column(name = "characteristic_value")
private String value;

and then execute (add to previous piece of code):

System.out.println("try to save");
    ClientProperty cliPro = new ClientProperty();
    cliPro.setClient(client);
    cliPro.setProperty(prop);
    cliPro.setValue("vocabulary");
    DAOInterface<ClientProperty> daocp = new HibernateDAOClientProperty();
    daocp.create(cliPro);

I magically get exactly what I want. Don't know how.

Hibernate: insert into client_characteristic_filled (id_client, id_characteristic, characteristic_value) values (?, ?, ?)
Hibernate: update client set client_activated=?, client_comment=?, client_name=? where client_id=?
Hibernate: update user_properties set property_name=?, property_type=? where property_id=?
Hibernate: update ref_client_to_prop set id_client=? where id_prop=?
Hibernate: insert into ref_client_to_prop (id_client, id_prop) values (?, ?)

Answer 1

In the first case, you're setting the book's student while inside a transaction, on a book that you just saved from the same transaction. The Book is thus attached to the session. So Hibernate tracks all the changes to its state, and when you commit, it notices that the book(s client has been set, and thus saves the association.

In the second case, you don't have any transaction. You create a property, and as soon as the DAO's transaction is done, the property becomes a detached object, ie a plain old object that Hibernate doesn't know about, whose state isn't tracked at all. Then you set the property's client on this detached object. But Hibernate doesn't know anything about that, and it thus doesn't insert the association.

Note that setting the properties of the client and saving the client doesn't save the association, because the owning side of the bidirectional association is Property.client , not Client.properties . And Hibernate only cares about the owning side.

Avoid using detached objects as much as you can. Start a transaction at the beginning of your business method, then use Hibernate, with only attached objects, then commit the transaction.