一对多
查询category中的某一条数据,同时查询该分类下的所有Product。
Category.java
public class Category { private Integer id; private String name; private List<Product> product; ....
product.java
public class Product { private Integer id; private String name; private String price; private Integer categoryId; ...
dao层
@Mapper public interface MyBatisDao { @Select("select * from product where categoryId = #{categoryId}") List<Product> findProductByCategoryId(Integer categoryId); @Results({ @Result(property = "id" , column = "id"), //products映射<List> Product,many=@Many是调用关联的方法,"id"是关联的条件, FetchType.LAZY是延迟加载 @Result(property = "products", column = "id", many=@Many(select = "com.david.dao.MyBatisDao.findProductByCategoryId",fetchType = FetchType.LAZY)) }) @Select("select * from category where id = #{id}") Category findCategoryById(Integer id); }
service层
@Service @Transactional public class MyBatisService { @Resource private MyBatisDao myBatisDao; public Category findCategoryById(){ Category category = myBatisDao.findCategoryById(1);return category; } }
测试:
一对一:把上面的@many=@Many换成one=@One即可
修改Product.java 增加属性
public class Product { private Integer id; private String name; private String price; private Integer categoryId; private Category category;
dao
@Results({ @Result(property = "id", column = "id"), @Result(property = "category", column = "categoryId",one=@One(select = "com.david.dao.MyBatisDao.findCategoryById",fetchType = FetchType.LAZY)) }) @Select("select * from Product where id = #{id}") Product findProductCategoryById(Integer id);
多对多:把多个字段映射成many=@Many即可
多对一:把上面dao方法的返回值从Category改成List<Category>
JAVA注解的局限性
返回多条Category
@Results({ @Result(property = "id" , column = "id"), //products映射<List> Product,many=@Many是调用关联的方法,"id"是关联的条件, FetchType.LAZY是延迟加载 @Result(property = "products", column = "id", many=@Many(select = "com.david.dao.MyBatisDao.findProductByCategoryId",fetchType = FetchType.LAZY)) }) @Select("select * from category") List<Category> findAllCategory();
category现在有三条记录,我要查出所有category及其对应的product,最终得到一个List<Category> 查询过程是这样的:
一共执行了四次查询,一次查category表,因为有三条记录,所以查了三次product表,以此来填充三个List<Product> products属性。如果category中又几百条数据,而且还有上十个表进行关联查询,结果无法想象。在传统的xml配置中,是可以用一条sql语句查出来的,不过mybatis官方做出了一个说明,由于java注解的局限性,不支持那种映射方式。所以,如果只想用一条sql语句查出关联映射,必须借助xml。
xml无限层嵌套映射
这里以三层嵌套为例,实现前端的三级菜单树。这是一个tree表,pid是其上级菜单的id。