@One 与 @Many 关联

MyBatis 注解模式下，关联查询通过 @One（一对一）和 @Many（一对多）配合 @Results 实现，替代 XML 中的 <association> 和 <collection>。

@One 一对一关联

@One 用于映射一对一关系，通过 select 属性指定子查询方法。

基本用法

public interface UserMapper {

    @Select("SELECT * FROM users WHERE id = #{id}")
    @Results({
        @Result(id = true, column = "id", property = "id"),
        @Result(column = "name", property = "name"),
        @Result(column = "dept_id", property = "department",
            one = @One(select = "com.example.mapper.DepartmentMapper.getById"))
    })
    User findUserWithDepartment(@Param("id") Long id);
}

子查询 Mapper：

public interface DepartmentMapper {

    @Select("SELECT * FROM departments WHERE id = #{id}")
    Department getById(@Param("id") Long id);
}

column 传参

column 属性指定将查询结果中的哪一列作为子查询参数：

多列传参示例：

@Result(column = "{deptId=dept_id, status=status}", property = "department",
    one = @One(select = "com.example.mapper.DepartmentMapper.getByCondition"))

对应子查询：

@Select("SELECT * FROM departments WHERE id = #{deptId} AND status = #{status}")
Department getByCondition(Map<String, Object> params);

@Many 一对多关联

@Many 用于映射一对多关系，返回类型为 List。

基本用法

public interface DepartmentMapper {

    @Select("SELECT * FROM departments WHERE id = #{id}")
    @Results({
        @Result(id = true, column = "id", property = "id"),
        @Result(column = "name", property = "name"),
        @Result(column = "id", property = "employees",
            many = @Many(select = "com.example.mapper.EmployeeMapper.getByDeptId"))
    })
    Department findDeptWithEmployees(@Param("id") Long id);
}

子查询 Mapper：

public interface EmployeeMapper {

    @Select("SELECT * FROM employees WHERE dept_id = #{deptId}")
    List<Employee> getByDeptId(@Param("deptId") Long deptId);
}

延迟加载

关联查询默认立即执行，可通过 fetchType 控制加载策略：

@Result(column = "id", property = "employees",
    many = @Many(
        select = "com.example.mapper.EmployeeMapper.getByDeptId",
        fetchType = FetchType.LAZY))

全局配置开启延迟加载：

mybatis:
  configuration:
    lazy-loading-enabled: true
    aggressive-lazy-loading: false

嵌套关联示例

一对一 + 一对多同时存在：

@Select("SELECT * FROM orders WHERE id = #{id}")
@Results({
    @Result(id = true, column = "id", property = "id"),
    @Result(column = "user_id", property = "user",
        one = @One(select = "com.example.mapper.UserMapper.getById")),
    @Result(column = "id", property = "items",
        many = @Many(select = "com.example.mapper.OrderItemMapper.getByOrderId"))
})
Order findOrderWithUserAndItems(@Param("id") Long id);

要点总结

• @One 映射一对一关系，返回单个对象；@Many 映射一对多关系，返回 List
• select 属性指定子查询方法的完全限定名，格式 接口全路径.方法名
• column 属性指定传递哪列数据给子查询，支持单列和多列（{key=column} 格式）
• fetchType 控制加载策略，建议关联查询使用 FetchType.LAZY 避免 N+1 性能问题
• 多表 JOIN 查询也可用 @Results 直接映射，无需子查询，性能更优