[英]JPA many to many relationship : zero or more
我是JPA的新手,所以我有一个关于多对多关系(零到更多实现)的问题:
如果您有类似这样的关系:
如前所述,一种产品可以不经订单即存在(通常它们会随着新产品的到货而被添加),之后可以在一项或多项订单上使用。 订单必须包含至少1个或更多产品。
你必须说明关系
@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
}
我想知道零对多关系是否仍然可以仅保留(目前)未链接到订单的产品?
但是,当订单使用产品时,应更新产品中的订单清单,也应更新orde中的产品清单。 我该如何执行此操作,或者JPA可以为我执行此操作?
您仍然可以保留产品而无需订购。
您必须手动更新关系的另一端。 JPA不会为您这样做。 (当然,如果您保存一个订单然后重新提取产品,则您的订单集合将被更新)
编辑来解释第二点:
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,您将得到类似:
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;
...
...
}
对? 还是我看错了
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