报错

报错内容如下

Expected one result (or null) to be returned by selectOne(), but found: 2

找原因

于是我就去看了一下源码

发现selectOne的实现是用的selectList查询到结果后在对list做判断

默认情况下如果list长度大于1就报错

    default T selectOne(@Param("ew") Wrapper<T> queryWrapper, boolean throwEx) {
        List<T> list = this.selectList(queryWrapper);
        int size = list.size();
        if (size == 1) {
            return list.get(0);
        } else if (size > 1) {
            if (throwEx) {
                throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + size);
            } else {
                return list.get(0);
            }
        } else {
            return null;
        }
    }

解决方案

方案一

通过源码可以看出第二个参数可出传入false跳过这个错误

    @Override
    public User getUser(String username) {
        User where = new User();
        where.setUsername(username);
        return userMapper.selectOne(new QueryWrapper<>(where), false);
    }

方案二

但是selectList方式还是符合我的心理预期的

于是我在网上找到了第二种方法

就是在queryWrapper上加limit 1

    @Override
    public User getUser(String username) {
        User where = new User();
        where.setUsername(username);
        QueryWrapper<User> userQueryWrapper = new QueryWrapper<>(where);
        userQueryWrapper.last("limit 1");
        return userMapper.selectOne(userQueryWrapper);
    }

方案三

虽然在queryWrapper上加手动加limit 1,但是要改的地方太多

于是可以使用切面来改进一下

@Aspect
@Component
public class MybatisAspect {

    // 配置织入点
    @Pointcut("execution(public * com.baomidou.mybatisplus.core.mapper.BaseMapper.selectOne(..))")
    public void selectOneAspect() {
    }

    @Before("selectOneAspect()")
    public void beforeSelect(JoinPoint point) {
        Object arg = point.getArgs()[0];
        if (arg instanceof AbstractWrapper) {
            ((AbstractWrapper<?, ?, ?>) arg).last("limit 1");
        }
    }
}