[英]Hibernate. How to properly organize relation a one-to-many with annotations?
I am trying to make a simple example of Hibernate. 我试图做一个简单的Hibernate示例。 I have two entities: User and Note. 我有两个实体:User和Note。 They have relation a one to many (one user can have a lot of notes). 它们具有一对多的关系(一个用户可以有很多笔记)。 Please help me to correctly display these relationships in a database using annotations.But I don't want to create third table for implementation of the relation. 请帮我使用注释在数据库中正确显示这些关系。但我不想创建第三个表来实现关系。 I need to have only two tables : 我只需要两张桌子 :
Here are my classes: 这是我的课程:
User.java : User.java :
@Entity
@Table(name = "user")
public class User {
@Id
@GeneratedValue
@Column(name = "id")
private Long id;
@Column(name = "name")
private String name;
@OneToMany(cascade = CascadeType.ALL, mappedBy="user") //Is it right value for mappedBy-parameter?
private List<Note> notes = new ArrayList<Note>();
// getters and setters
Note.java : Note.java :
@Entity
@Table(name = "note")
public class Note {
@Id
@GeneratedValue
@Column(name = "id")
private Long id;
@Column(name = "content")
private String content;
@ManyToOne
private User user;
// getters and setters
Main.java : Main.java :
public static void main(String[] args) {
Session session = HibernateUtil.getSessionFactory().openSession();
Transaction transaction = null;
try {
transaction = session.beginTransaction();
List<Note> notes = new ArrayList<Note>();
Note note1 = new Note();
note1.setContent("my first note");
Note note2 = new Note();
note2.setContent("my second note");
notes.add(note1);
notes.add(note2);
User user = new User();
user.setName("Andrei");
user.setNotes(notes);
session.save(user);
transaction.commit();
} catch (HibernateException e) {
transaction.rollback();
e.printStackTrace();
} finally {
session.close();
}
}
hibernate.cfg.xml : hibernate.cfg.xml :
<property name="dialect">org.hibernate.dialect.MySQLInnoDBDialect</property>
<property name="connection.pool_size">1</property>
<property name="current_session_context_class">thread</property>
<property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>
<property name="show_sql">true</property>
<property name="hbm2ddl.auto">create-drop</property>
<mapping class="com.vaannila.blog.User" />
<mapping class="com.vaannila.blog.Note" />
After executing this code in my database Hibernate has created and has filled two tables: 在我的数据库中执行此代码后,Hibernate创建并填充了两个表:
But I encounter a problem: the field user_id value is null in note table.Although it must be equal to the user id (in this case 1). 但是我遇到了一个问题:在note表中字段user_id值为null。虽然它必须等于用户id(在这种情况下为1)。
What do I need to add to annotations for to solve this problem and this example to work correctly? 我需要添加什么才能解决此问题以及此示例才能正常工作? But without creating additional tables. 但无需创建其他表。
I would really appreciate any help! 我真的很感激任何帮助!
You must set the User
inside each note as you have defined a bidirectional relationship. 您已定义双向关系,因此必须在每个注释中设置User
。 Instead of letting clients pass in a list of notes directly, create User.addNote
and have it set the relationship correctly. 不要让客户端直接传入注释列表,而是创建User.addNote
并让它正确设置关系。
class User {
...
public void addNote(Note note) {
note.user = this;
notes.add(note);
}
}
Your test code thus becomes 你的测试代码就这样了
Note note1 = new Note();
note1.setContent("my first note");
Note note2 = new Note();
note2.setContent("my second note");
User user = new User();
user.setName("Andrei");
user.addNote(note1);
user.addNote(note2);
session.save(user);
You can further improve this by adding the basic fields to the constructors of your objects simplifying the above to 您可以通过将基本字段添加到对象的构造函数来进一步改进这一点,从而简化上述内容
User user = new User("Andrei");
user.addNote(new Note("my first note"));
user.addNote(new Note("my second note"));
session.save(user);
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.