解决mybatis 中collection嵌套collection引发的bug

我就废话不多说了，大家还是直接看代码吧~

  
  
  
 
    
  

 

  
  
  
  
  
 
    
 

 

  
  
  
  

 

  p.Id, p.Name, p.SurName,
 
 

  c.Id as c_Id, c.ParentId as c_ParentId, c.Name as c_Name, c.SurName as c_Surname, c.Age as c_Age,

 

  t.Id as t_Id, t.Name as t_Name, t.Color as t_Color
 
 

  select 
  
  
  
  from Parent p
 
  left outer join Child c on p.Id = c.ParentId
  left outer join Toy t on c.Id = t.ChildId
  where p.id = #{id,jdbcType=VARCHAR}

表面来看没有任何问题 实际 查询的child对象中的toys一直是空

类关系介绍：

Parent类有属性ArrayList children

Child类有属性ArrayList toys

Toy是一个普通的类

原因在于：

columnPrefix配置的是t_实际mybatis处理后是 c_t_

解决办法：

只需要修改 sql 修改前缀为 c_t_ 即可

  t.Id as c_t_Id, t.Name as c_t_Name, t.Color as c_t_Color
 

补充知识：mybatis 嵌套的结果集不能被安全的转为自定义ResultHandler 的解决办法

org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.executor.ExecutorException: Mapped Statements with nested result mappings cannot be safely used with a custom ResultHandler. Use safeResultHandlerEnabled=false setting to bypass this check.

问题描述

session.select("dao.ArticleMapper.selectAll", null, new RowBounds(1, 2),resultHandler);

会报不安全, 查询Configuration 源码发现里面有一个常量是

public Configuration() {
  this.safeRowBoundsEnabled = false;
  this.safeResultHandlerEnabled = true;//意思是不允许自定义ResultHand 处理器, 
  this.mapUnderscoreToCamelCase = false;
  this.aggressiveLazyLoading = true;

解决办法

  public static SqlSession getsqlSession(){
  SqlSession session = sqlSessionFactory.openSession(ExecutorType.REUSE);
  Configuration configuration = session.getConfiguration(); //反射得到configuration ,然后
  configuration.setSafeResultHandlerEnabled(false); // 设置为false
  return session;
}

这样就可以了。

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