多对多用左连接发送两个查询[重复]

Question

我的实体是用户和角色。它们之间存在着多对多的关系。角色的获取在用户中是惰性的。

@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(name = "users_roles",
joinColumns = @JoinColumn(name = "user_id"),
inverseJoinColumns = @JoinColumn(name = "role_id"))
@BatchSize(size = 3)
private Set<Role> roles = new HashSet<>();

我想在一个查询中得到用户和规则数据。所以我试着用JPQL得到这些数据。我的用户库是

@Query(value = "select u from User u left join u.roles where u.username=:username")
Optional<User> findByUserNameWithRoles(@Param("username") String username);

我记录我的查询与数据源代理，我有两个查询，但我没有离开加入一个查询。为什么不做一个查询？

我的日志是

Name:Db Connection, Connection:4, Time:1, Success:True
Type:Prepared, Batch:False, QuerySize:1, BatchSize:0
Query:["select user0_.id as id1_1_, user0_.created_by as created_2_1_, user0_.created_date as created_3_1_, user0_.last_modified_by as last_mod4_1_, user0_.last_modified_date as last_mod5_1_, user0_.email as email6_1_, user0_.first_name as first_na7_1_, user0_.password as password8_1_, user0_.status as status9_1_, user0_.surname as surname10_1_, user0_.user_uuid as user_uu11_1_, user0_.username as usernam12_1_ from users user0_ left outer join users_roles roles1_ on user0_.id=roles1_.user_id left outer join roles role2_ on roles1_.role_id=role2_.id where user0_.username=?"]
Params:[(yusuf)] 
- 

Name:Db Connection, Connection:4, Time:1, Success:True
Type:Prepared, Batch:False, QuerySize:1, BatchSize:0
Query:["select roles0_.user_id as user_id1_2_1_, roles0_.role_id as role_id2_2_1_, role1_.id as id1_0_0_, role1_.role_name as role_nam2_0_0_ from users_roles roles0_ inner join roles role1_ on roles0_.role_id=role1_.id where roles0_.user_id=?"]
Params:[(1)]

Answer 1

第一个查询用于收集有关用户的信息。第二个查询是获取链接到该用户的所有角色。这就是所谓的n1ORM问题，你可以在这里和这里阅读

解决方案是使用join-fetch而不是left-join