[英]QueryDSL, Hibernate, JPA — using .fetchJoin() and getting data in first SELECT, so why N+1 queries after?
[英]QueryDSL fetchJoin does not fetch the data
我正面临这个问题:
假设我有 3 个这样的实体:
实体 A:
long id
String someField
// No bidirectional linkage to B entity via hibernate
实体 B:
long id
String someBField
@ManyToOne(optional = false, fetch = FetchType.LAZY)
@JoinColumn(name="b_id")
A entityA
@ManyToOne(optional = true, fetch = FetchType.LAZY)
@JoinColumn(name="b_id")
C entityC;
实体 C:
long id
String someCField
// No bidirectional linkage to B entity via hibernate
现在,目标是(为简单起见,有一些排序和过滤,但不影响我的问题)返回所有 B 记录,每一条都带有 A 和 C 记录
所以我正在做这样的事情(我习惯于使用 spring-data-jpa 来(左)JOIN FETCH 属性以避免按需延迟加载以防止将无用的查询触发到数据库中,我想在 QueryDSL 中做完全相同的事情)
JPAQuery<DealBo> query = new JPAQuery<>(entityManager);
query.select(qB)
.from(qB)
.innerJoin(qA).on(qA.a_id.eq(qB.id)).fetchJoin()
.innerJoin(qC).on(qC.a_id.eq(qB.id)).fetchJoin()
.fetch()
And I expect one SQL where in select clause there are data from all 3 tables (entities), where QueryDSL (or Hibernate, I am not completely sure what tool will do SQL -> Entity mapping) map the result to Entity objects. 但我真正得到的只是 select 之类的
select b.id, b.someBfield from b
inner join a // join clause is right and omitted for simplicity
inner join b // join clause is right and omitted for simplicity
因此,当我调用一项时,QueryDSL 例如返回了什么
b.getC() 或 b.getA(),我正在向数据库发起另一个查询,这是我首先要避免的事情。
我究竟做错了什么?
我认为,连接条件的定义是不合适的。
希望我已经用 UserEntity <- UserRoleEntity -> RoleEntity 重新创建了描述的星座:
@Entity
@Table(name = "t_user")
public class UserEntity {
@Id
@Column(name = "id")
private Integer id;
@Column(name = "name")
// ..
}
@Entity
@Table(name = "t_user_role")
public class UserRoleEntity {
@Id
@Column(name = "id")
private Integer id;
@ManyToOne
@JoinColumn(name = "user_id")
private UserEntity user;
@ManyToOne
@JoinColumn(name = "role_id")
private RoleEntity role;
// ..
}
@Entity
@Table(name = "t_role")
public class RoleEntity {
@Id
@Column(name = "id")
private Integer id;
@Column(name = "name")
private String name;
// ..
}
查询
List<UserRoleEntity> findAll() {
JPAQuery<UserRoleEntity> query = new JPAQuery<>(entityManager);
return query.select(QUserRoleEntity.userRoleEntity)
.from(QUserRoleEntity.userRoleEntity)
.innerJoin(QUserRoleEntity.userRoleEntity.user).fetchJoin()
.innerJoin(QUserRoleEntity.userRoleEntity.role).fetchJoin()
.fetch();
}
获取关联的表,并且对用户关联的后续迭代不会从数据库加载用户实体。
生成的 SQL 看起来像
select
userroleen0_.id as id1_5_0_,
userentity1_.id as id1_4_1_,
roleentity2_.id as id1_2_2_,
userroleen0_.role_id as role_id2_5_0_,
userroleen0_.user_id as user_id3_5_0_,
userentity1_.name as name2_4_1_,
roleentity2_.name as name2_2_2_
from t_user_role userroleen0_
inner join t_user userentity1_ on userroleen0_.user_id=userentity1_.id
inner join t_role roleentity2_ on userroleen0_.role_id=roleentity2_.id
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