[英]JPA many to many relationship : zero or more
我是JPA的新手,所以我有一個關於多對多關系(零到更多實現)的問題:
如果您有類似這樣的關系:
如前所述,一種產品可以不經訂單即存在(通常它們會隨着新產品的到貨而被添加),之后可以在一項或多項訂單上使用。 訂單必須包含至少1個或更多產品。
你必須說明關系
@Entity(name = "ORDERS")
public class Order {
@Id
@Column(name = "ORDER_ID", nullable = false)
@GeneratedValue(strategy = GenerationType.AUTO)
private long orderId;
@Column(name = "CUST_ID")
private long custId;
@Column(name = "TOTAL_PRICE", precision = 2)
private double totPrice;
@ManyToMany(fetch=FetchType.EAGER)
@JoinTable(name="ORDER_DETAIL",
joinColumns=
@JoinColumn(name="ORDER_ID", referencedColumnName="ORDER_ID"),
inverseJoinColumns=
@JoinColumn(name="PROD_ID", referencedColumnName="PROD_ID")
)
private List<Product> productList;
...............
The other attributes and getters and setters goes here
}
和
@Entity(name = "PRODUCT")
public class Product {
@Id
@Column(name = "PROD_ID", nullable = false)
@GeneratedValue(strategy = GenerationType.AUTO)
private long prodId;
@Column(name = "PROD_NAME", nullable = false,length = 50)
private String prodName;
@Column(name = "PROD_DESC", length = 200)
private String prodDescription;
@Column(name = "REGULAR_PRICE", precision = 2)
private String price;
@Column(name = "LAST_UPDATED_TIME")
private Date updatedTime;
@ManyToMany(mappedBy="productList",fetch=FetchType.EAGER)
private List<Order> orderList;
...............
The other attributes and getters and setters goes here
}
我想知道零對多關系是否仍然可以僅保留(目前)未鏈接到訂單的產品?
但是,當訂單使用產品時,應更新產品中的訂單清單,也應更新orde中的產品清單。 我該如何執行此操作,或者JPA可以為我執行此操作?
您仍然可以保留產品而無需訂購。
您必須手動更新關系的另一端。 JPA不會為您這樣做。 (當然,如果您保存一個訂單然后重新提取產品,則您的訂單集合將被更新)
編輯來解釋第二點:
Product persitentProduct = ... //some product
Order newOrder = new Order();
newOrder.getProducts().add(persitentProduct);
//at this point : persistentProduct.getOrders().contains(newOrder)==false
entityManager.persist(newOrder);
//at this point nothing has changed on the other side of the relationship:
// i.e. : persistentProduct.getOrders().contains(newOrder)==false
然后,您將得到類似:
public class Order {
private List products;
...
public void addProduct(Product product) {
this.products.add(product);
if !(product.getOrders().contains(product) {
product.getOrders().add(this);
}
}
...
}
public class Product {
private List orders;
...
...
}
對? 還是我看錯了
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