我正在尝试在Spring Boot中使用嵌套的投影。我有2个实体，Parent和Child，而Parent与@OneToMany具有单向Child关系。
以下是类：（使用龙目岛注释）
@Entity
@Data @NoArgsConstructor
public class Parent {
@Id
@GeneratedValue
private long id;
private String basic;
private String detail;
@OneToMany(fetch = FetchType.EAGER)
private List<Child> children;
public Parent(String basic, String detail, List<Child> children) {
this.basic = basic;
this.detail = detail;
this.children = children;
}
}
@Entity
@Data @NoArgsConstructor
public class Child {
@Id
@GeneratedValue(strategy = GenerationType.TABLE)
private long id;
private String basic;
private String detail;
public Child(String basic, String detail) {
this.basic = basic;
this.detail = detail;
}
}
在即时获取数据而不进行投影时，我得到以下信息：
[
{
"id": 1,
"basic": "parent-basic-1",
"detail": "parent-detail-1",
"children": [
{
"id": 1,
"basic": "child-basic-1",
"detail": "child-detail-1"
},
{
"id": 2,
"basic": "child-basic-2",
"detail": "child-detail-2"
}
]
},
{
"id": 2,
"basic": "parent-basic-2",
"detail": "parent-detail-2",
"children": [
{
"id": 3,
"basic": "child-basic-3",
"detail": "child-detail-3"
},
{
"id": 4,
"basic": "child-basic-4",
"detail": "child-detail-4"
}
]
}
目标是：
{
"id": 1,
"basic": "parent-basic-1",
"children": [1,2]
},
{
"id": 2,
"basic": "parent-basic-2",
"children": [3,4]
}
但是，实现这一目标似乎完全不可能。
@Value
public class ParentDto {
long id;
String basic;
// wanted to get it to work with just Child instead of ChildDto first, before getting ChildDto to work
Collection<Child> children;
public ParentDto(long id, String basic, Collection<Child> children) {
this.id = id;
this.basic = basic;
this.children = children;
}
}
// Constructor Projection in Repository
@Query("select new whz.springbootdemo.application.constructor_projection.ParentDto(p.id, p.basic, p.children) from Parent p")
List<ParentDto> findAllConstructorProjected();
但这会导致以下错误：
could not prepare statement; SQL [select parent0_.id as col_0_0_, parent0_.basic as col_1_0_, . as col_2_0_ from parent parent0_ inner join parent_children children1_ on parent0_.id=children1_.parent_id inner join child child2_ on children1_.children_id=child2_.id]; nested exception is org.hibernate.exception.SQLGrammarException: could not prepare statement
// Dynamic Projection in Repository
List<ParentDto> findAllDynamicProjectionBy();
导致以下错误：
org.hibernate.hql.internal.ast.QuerySyntaxException:
Unable to locate appropriate constructor on class [whz.springbootdemo.application.constructor_projection.ParentDto].
Expected arguments are: <b>long, java.lang.String, whz.springbootdemo.application.child.Child</b>
[select new whz.springbootdemo.application.constructor_projection.ParentDto(generatedAlias0.id, generatedAlias0.basic, children) from whz.springbootdemo.application.parent.Parent as generatedAlias0 left join generatedAlias0.children as children]; nested exception is java.lang.IllegalArgumentException: org.hibernate.hql.internal.ast.QuerySyntaxException: Unable to locate appropriate constructor on class [whz.springbootdemo.application.constructor_projection.ParentDto]. Expected arguments are: long, java.lang.String, whz.springbootdemo.application.child.Child [select new whz.springbootdemo.application.constructor_projection.ParentDto(generatedAlias0.id, generatedAlias0.basic, children) from whz.springbootdemo.application.parent.Parent as generatedAlias0 left join generatedAlias0.children as children]
基本上告诉我执行了连接，但是值arent由父代的id分组，因此导致x行，其中x是父代的孩子数，每个子代都有父子的基本信息和一个其子项信息。
// Interface Projection in Repository
List<ParentDtoInterface> findAllInterfaceProjectedBy();
public interface ParentDtoInterface {
long getId();
String getBasic();
List<ChildDtoInterface> getChildren();
}
public interface ChildDtoInterface {
long getId();
}
结果为：
[
{
"id": 1,
"children": [
{
"id": 1
},
{
"id": 2
}
],
"basic": "parent-basic-1"
},
{
"id": 2,
"children": [
{
"id": 3
},
{
"id": 4
}
],
"basic": "parent-basic-2"
}
]
现在我对Interface-Projection的问题是，它不仅会加载预期的属性，而且会加载所有属性，但是jackson只会序列化Interface提供的属性，因为它使用了Class / Interface-Definition。
已加载父项：（sql日志；请参见第4行，已加载详细信息）
select
parent0_.id as id1_1_,
parent0_.basic as basic2_1_,
parent0_.detail as detail3_1_
from
parent parent0_
接口投影似乎也真的很慢（请参阅this Stackoverflow question），我仍然必须解开子项的包装，因为它们被指定为[{id：1}，{id：2}]，但我确实需要[1,2]。我知道我可以使用@JsonIdentityReference(alwaysAsId = true)来做到这一点，但这只是一种解决方法。
我也有点困惑为什么在n + 1个查询中加载数据-父母中有1个，每个父母孩子中有n个（其中n是父母的数量）：
select
parent0_.id as id1_1_,
parent0_.basic as basic2_1_,
parent0_.detail as detail3_1_
from
parent parent0_
select
children0_.parent_id as parent_i1_2_0_,
children0_.children_id as children2_2_0_,
child1_.id as id1_0_1_,
child1_.basic as basic2_0_1_,
child1_.detail as detail3_0_1_
from
parent_children children0_
inner join
child child1_
on children0_.children_id=child1_.id
where
children0_.parent_id=?
//... omitting further child queries
我尝试过@OneToMany(fetch=FetchType.LAZY)和@Fetch(FetchType.JOINED)-都得到与上述相同的结果。
所以主要问题是：有什么方法可以使用Spring Boot来实现嵌套实体的投影，以便仅在尽可能少的查询中加载所需的数据，在最佳情况下，我可以对其进行调整这样，我不必加载List子项，而只需加载List childIds（也许通过一个Jpa查询，该查询通过父项对连接的行进行分组，并可以从Child中提取所需的数据？）。
我正在使用Hibernate和内存数据库。
感谢您的回答或提示！
编辑：澄清一下：我并不是想找到一种以所需格式序列化数据的方法-这已经可以实现了。主要重点是仅从数据库中加载必要的信息。
答案 0 :(得分：0)
这将始终获取孩子，但可以给您想要的结果。
public interface SimpleParentProjection {
String getBasic();
String getDetail();
@Value("#{T(SimpleParentProjection).toId(target.getChildren())}")
String[] getChildren();
static String[] toId(Set<Child> childSet) {
return childSet.stream().map(c -> String.valueOf(c.getId())).toArray(String[]::new);
}
}