# 23.3\. Customer(客户)/Order(订单)/Product(产品) 现在来考虑`Customer`,`Order` ， `LineItem` 和 `Product`关系的模型。`Customer` 和 `Order`之间 是一对多的关系，但是我们怎么来描述`Order` / `LineItem` / `Product`呢？ 我可以把`LineItem`作为描述`Order` 和 `Product` 多对多关系的关联类，在Hibernate，这叫做组合元素。  映射文件如下： ``` <hibernate-mapping> <class name="Customer" table="customers"> <id name="id"> <generator class="native"/> </id> <property name="name"/> <set name="orders" inverse="true"> <key column="customer_id"/> <one-to-many class="Order"/> </set> </class> <class name="Order" table="orders"> <id name="id"> <generator class="native"/> </id> <property name="date"/> <many-to-one name="customer" column="customer_id"/> <list name="lineItems" table="line_items"> <key column="order_id"/> <list-index column="line_number"/> <composite-element class="LineItem"> <property name="quantity"/> <many-to-one name="product" column="product_id"/> </composite-element> </list> </class> <class name="Product" table="products"> <id name="id"> <generator class="native"/> </id> <property name="serialNumber"/> </class> </hibernate-mapping> ``` `customers`, `orders`, `line_items` 和 `products` 分别保存着customer, order, order line item 和 product的数据。 `line_items`也作为连接orders 和 products的关联表。 ``` create table customers ( id BIGINT not null generated by default as identity, name VARCHAR(255), primary key (id) ) create table orders ( id BIGINT not null generated by default as identity, customer_id BIGINT, date TIMESTAMP, primary key (id) ) create table line_items ( line_number INTEGER not null, order_id BIGINT not null, product_id BIGINT, quantity INTEGER, primary key (order_id, line_number) ) create table products ( id BIGINT not null generated by default as identity, serialNumber VARCHAR(255), primary key (id) ) alter table orders add constraint ordersFK0 foreign key (customer_id) references customers alter table line_items add constraint line_itemsFK0 foreign key (product_id) references products alter table line_items add constraint line_itemsFK1 foreign key (order_id) references orders ```