Mybatis关联关系

1、一对多，以订单与订单项为例：

1-1：将数据导入数据库
1-2：通过mybatis-generator插件生成dao、mapper、model
1-3：实体类建立关联关系：
1-3-1、一对多，订单的实体类：一个订单对应多个订单项

private Set<OrderItem> orderItems = new HashSet<>();

1-3-2、多对一，订单项的实体类：多个订单项对应一个订单

private Order order;

1-4：配置mybatis关联映射：
1-4-1：一对多：订单中的Mapper.xml配置

<collection property= oftype=“类的全路径名”>

<!--配置，与数据库保持一致-->
<!--订单配置-->
  <resultMap id=\"orderMap\" type=\"com.zking.ssm.model.Order\">
    <result property=\"orderId\" column=\"order_id\"></result>
    <result property=\"orderNo\" column=\"order_no\"></result>
    <!--一对多     订单项配置-->
   <!--  property: 指的是集合属性的值, ofType：指的是集合中元素的类型 -->
    <collection property=\"orderItems\" ofType=\"com.zking.ssm.model.OrderItem\">
      <result property=\"orderItemId\" column=\"order_item_id\"></result>
      <result property=\"productId\" column=\"product_id\"></result>
      <result property=\"quantity\" column=\"quantity\"></result>
      <result property=\"oid\" column=\"oid\"></result>
    </collection>
  </resultMap>

1-4-2：多对一：订单项中的Mapper.xml配置：

<assocation property javaType = \" 类的全路径名\"

 <resultMap id=\"OrderItemMap\" type=\"com.zking.ssm.model.OrderItem\">
    <result property=\"orderItemId\" column=\"order_item_id\"></result>
    <result property=\"productId\" column=\"product_id\"></result>
    <result property=\"quantity\" column=\"quantity\"></result>
    <result property=\"oid\" column=\"oid\"></result>
       <!-- 多对一的关系 -->
        <!-- property: 指的是属性的值, javaType：指的是属性的类型-->
    <association property=\"order\" javaType=\"com.zking.ssm.model.Order\">
      <result property=\"orderId\" column=\"order_id\"></result>
      <result property=\"orderNo\" column=\"order_no\"></result>
    </association>
  </resultMap>
  

1-5：订单中方法的配置：
1-5-1：根据订单id去查询：

 List<Order> queryOrderById(Integer orderId);

1-5-2：Mapper.xml配置：

 <select id=\"queryOrderById\" resultMap=\"orderMap\" parameterType=\"java.lang.Integer\">
    select * from t_hibernate_order o,t_hibernate_order_item oi
    where o.order_id = oi.oid
    and o.order_id=#{orderId}

  </select>

1-6：订单项中方法的配置：

注意：使用左外连接而非内连接！！！

1-6-1：根据订单项id去查询：

  OrderItem queryOrderItemById(Integer orderItemId);

1-6-2：Mapper.xml配置：

<select id=\"queryOrderItemById\" resultMap=\"OrderItemMap\" parameterType=\"java.lang.Integer\">
    select * from t_hibernate_order o,t_hibernate_order_item oi
    where o.order_id = oi.oid
    and oi.order_item_id=#{orderItemId}

  </select>

1-7：Service配置：
1-7-1：订单：OrderService:

public interface OrderService {

    List<Order> queryOrderById(Integer orderId);
}

1-7-2:订单：OrderServiceImpl：

@Service
public class OrderServiceImpl implements OrderService {

    @Autowired
    private OrderMapper orderMapper;

    @Override
    public List<Order> queryOrderById(Integer orderId) {
        return orderMapper.queryOrderById(orderId);
    }
}

1-7-3：测试类：OrderServiceTest （订单测试类）

public class OrderServiceTest extends SpringJunitBaseTest {

    @Autowired
    private OrderService orderService;

    @Test
    public void queryOrderById() {
        //查询订单编号为3 的订单
        List<Order> orders = orderService.queryOrderById(3);
        for (Order order : orders) {
            System.out.println(order);
        }
    }
}

结果：

2、多对多：书籍与类别为例：

2-1：书籍与类别是一对多的关系
书籍实体类配置：

    private Set<Category> categories = new HashSet<>();

2-2：类别与书籍也是一对多的关系：
类别实体类配置：

 private Set<HBook> hBooks = new HashSet<>();

2-3：配置mybatis关联映射：
2-3-1：一对多、书籍中的Mapper.xml配置：

<collection property= oftype=“类的全路径名”>

<resultMap id=\"bookMap\" type=\"com.zking.ssm.model.HBook\">
    <result property=\"bookId\" column=\"book_id\"></result>
    <result property=\"bookName\" column=\"book_name\"></result>
    <result property=\"price\" column=\"price\"></result>
    
    <collection property=\"categories\" ofType=\"com.zking.ssm.model.Category\">
      <result property=\"categoryId\" column=\"category_id\"></result>
      <result property=\"categoryName\" column=\"category_name\"></result>
    </collection>
  </resultMap>

2-3-2：一对多、类别中的Mapper.xml配置：

<collection property= oftype=“类的全路径名”>

 <resultMap id=\"CategoryMap\" type=\"com.zking.ssm.model.Category\">
    <result property=\"categoryId\" column=\"category_id\"></result>
    <result property=\"categoryName\" column=\"category_name\"></result>
    
    <collection property=\"hBooks\" ofType=\"com.zking.ssm.model.HBook\">
      <result property=\"bookId\" column=\"book_id\"></result>
      <result property=\"bookName\" column=\"book_name\"></result>
      <result property=\"price\" column=\"price\"></result>
    </collection>
  </resultMap>

2-4：方法配置：
2-4-1：HBookMapper配置

 HBook queryBookById(Integer bookId);

2-4-2：Mapper.xml配置：

  <select id=\"queryBookById\" resultMap=\"bookMap\" parameterType=\"java.lang.Integer\">
    select * from t_hibernate_book b,t_hibernate_book_category bc,t_hibernate_category c
    where b.book_id = bc.bid
    and bc.cid=c.category_id
    and b.book_id = #{bookId}
  </select>

2-5：Service配置：
2-5-1：HBookService

public interface HBookService {

    HBook queryBookById(Integer bookId);
}

2-5-2：HBookServiceImpl 
@Service
public class HBookServiceImpl implements HBookService {

    @Autowired
    private HBookMapper hBookMapper;

    @Override
    public HBook queryBookById(Integer bookId) {
        return hBookMapper.queryBookById(bookId);
    }
}

2-6：测试：

public class HBookServiceTest extends SpringJunitBaseTest {

    @Autowired
    private  HBookService hBookService;

    @Test
    public void queryBookById() {
        HBook hBook = hBookService.queryBookById(2);
        System.out.println(hBook);

    }