JPA Learning Notes (8)--mapping a one-to-many association relationship

One-to-many correlation relationship

This article has many and many to one is the same, therefore will not write very detailed. If you don't understand, you can refer to the JPA study notes (7)--mapping many-to-one relationships

Order entity Class

 PackageCom.jpa.helloworld2;ImportJavax.persistence.Column;Importjavax.persistence.Entity;ImportJavax.persistence.FetchType;ImportJavax.persistence.GeneratedValue;ImportJavax.persistence.GenerationType;ImportJavax.persistence.Id;ImportJavax.persistence.JoinColumn;ImportJavax.persistence.ManyToOne;Importjavax.persistence.Table;ImportJavax.persistence.TableGenerator;@Table(name="T_order")@Entity Public  class Order {    @Column(name="ID")@GeneratedValue    @Id    PrivateInteger ID;@Column(name="Order_name")PrivateString Ordername; PublicIntegergetId() {returnId } Public void setId(Integer ID) { This. id = ID; } PublicStringGetordername() {returnOrdername; } Public void Setordername(String ordername) { This. ordername = Ordername; }@Override     PublicStringtoString() {return "Order [id="+ ID +", ordername="+ Ordername +"]"; }}

User entity class

 PackageCom.jpa.helloworld2;ImportJava.util.ArrayList;ImportJava.util.List;ImportJavax.persistence.Column;Importjavax.persistence.Entity;ImportJavax.persistence.GeneratedValue;ImportJavax.persistence.GenerationType;ImportJavax.persistence.Id;ImportJavax.persistence.JoinColumn;ImportJavax.persistence.OneToMany;Importjavax.persistence.Table;@Table(name="T_user")@Entity Public  class User {    @Column(name="ID")@GeneratedValue(Strategy=generationtype.auto)@Id    PrivateInteger ID;@Column(name="NAME")PrivateString name;@JoinColumn(name="user_id")@OneToMany    Privatelist<order> orders =NewArraylist<> (); PublicIntegergetId() {returnId } Public void setId(Integer ID) { This. id = ID; } PublicStringGetName() {returnName } Public void SetName(String name) { This. name = name; } PublicList<order>getorders() {returnOrders } Public void setorders(list<order> orders) { This. orders = orders; }@Override     PublicStringtoString() {return "User [id=]+ ID +", name="+ name +", orders="+ orders +"]"; }}

A user can have more than one order, and in user there is a property List<Order> orders , which is a one-to-many association. It's a little different from many to one.

One-to-many use @onetomany annotations, and foreign-key relationships use @joincolumn (name= "user_id").

Note: The @joincolumn (name= "user_id"), although written in the USER entity class, is added to the T_order table in the "user_id" field.

Insert

Order Order1 = New Order ();Order1. Setordername("O1");Order Order2 = New Order ();Order2. Setordername("O2");User user = new user ();User. SetName("Tom");User. GetOrders(). Add(Order1);User. GetOrders(). Add(Order2);Insert Entitymanager. Persist(Order1);Entitymanager. Persist(Order2);Entitymanager. Persist(user);

Here the three objects are inserted in the order in which the results are reversed. This is a bit different from many to one.

Inquire

System.out.println(entityManager.find(User.class,1));

A one-to-many query that uses lazy-loaded loading policies By default, you can use the @OneToMany(fetch=FetchType.EAGER) change policy

Delete

User user2 = entityManager.find(User.class1);entityManager.remove(user2);

In many-to-one, this deletion throws an exception because of a foreign key constraint, but the pair is a bit different.

As you can see, it is to place the user_id field blank. But this is obviously not what we want.

In fact, we can use the Cascade property to change the policy, for example, cascade Delete.

@OneToMany(cascade=CascadeType.REMOVE)

Cascade Property There are other values, we Baidu or read the document.

Modify

User u = entityManager.find(User.class2);u.getOrders().get(0).setOrderName("orderName");

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JPA Learning Notes (8)--mapping a one-to-many association relationship