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我不明白映射雙向列表時Hibernate的行爲。 Hibernate產生的SQL語句對我來說似乎並不是最優的。有人可以啓發我嗎？用休眠映射雙向列表
該場景如下：我有一對多的親子關係。我將這種關係映射到雙向列表。
按照Hibernate Annotation Reference Guide（章：使用索引的集合雙向關聯）的映射應該是這樣的：
@Entity
public class Parent {
@Id @GeneratedValue private long id;
@Version private int version;
private String name;
@OneToMany(cascade = CascadeType.ALL)
@JoinColumn(name = "parent_id", nullable=false)
@org.hibernate.annotations.IndexColumn(name = "parent_index")
List<Child> children = new ArrayList<Child>();
...
@Entity
public class Child {
@Id @GeneratedValue private Long id;
@Version private int version;
private String name;
@ManyToOne
@JoinColumn(name = "parent_id", updatable = false, insertable = false, nullable=false)
private Parent parent;
...
但在這種情況下，休眠產生持續一個孩子父母時，三個SQL語句：
Hibernate: insert into Parent (name, version, id) values (?, ?, ?)
Hibernate: insert into Child (name, price, version, parent_id, parent_index, id) values (?, ?, ?, ?, ?, ?)
Hibernate: update Child set parent_id=?, parent_index=? where id=?
第三條語句似乎是多餘的，因爲parent_id和parent_index似乎已經在第二條語句中設置。
當我改變映射和重複屬性「更新=假，可插入=假」到@JoinColumn的聲明在家長是這樣的：
@Entity
public class Parent {
@Id @GeneratedValue private long id;
@Version private int version;
private String name;
@OneToMany(cascade = CascadeType.ALL)
@JoinColumn(name = "parent_id", updatable = false, insertable = false, nullable=false)
@org.hibernate.annotations.IndexColumn(name = "parent_index")
List<Child> children = new ArrayList<Child>();
...
@Entity
public class Child {
@Id @GeneratedValue private Long id;
@Version private int version;
private String name;
@ManyToOne
@JoinColumn(name = "parent_id", updatable = false, insertable = false, nullable=false)
private Parent parent;
...
...然後休眠似乎產生更優化的SQL：
Hibernate: insert into Parent (name, version, id) values (?, ?, ?)
Hibernate: insert into Child (name, price, version, parent_id, parent_index, id) values (?, ?, ?, ?, ?, ?)
客戶端代碼如下所示：
EntityManagerFactory emf = Persistence.createEntityManagerFactory("test");
EntityManager em = emf.createEntityManager();
EntityTransaction tx = em.getTransaction();
tx.begin();
Parent newParent = new Parent();
newParent.setName("Parent1");
Child newChild = new Child();
newChild.setName("Child1");
newParent.getChildren().add(newChild);
newChild.setParent(newParent);
em.persist(newParent);
em.flush();
tx.commit();
我正在使用hibernate-entitymanager 3.4.0.GA.
我錯過了什麼？ 「Hibernate參考指南」不正確，還是我忽略了某些內容？