How to handle Spring Boot/ Spring Data projections with entity relationships (nested projection) - spring

Question

I'm trying to get nested projections working in Spring Boot. I have 2 entities, Parent and Child, wheras Parent has a unidirectional #OneToMany relationship to Child.
Here are the classes: (using Lombok-Annotations)
#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;
}
}
When im fetching the data without projecting i get the following:
[
{
"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"
}
]
}
and the goal would be the following:
{
"id": 1,
"basic": "parent-basic-1",
"children": [1,2]
},
{
"id": 2,
"basic": "parent-basic-2",
"children": [3,4]
}
However it seems completly impossible to achive this.
So far I've tried Constructor Projection:
#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();
but that leads to the following error:
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
Trying Dynamic Projection:
// Dynamic Projection in Repository
List<ParentDto> findAllDynamicProjectionBy();
leads to the following error:
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]
which basically tells me that a join is executed, but the values arent grouped by the id of the parent, thus resulting in x rows, where x is the number of childs the parents has, each with the parents basic information and one of its childs information.
The only thing "working" is Interface Projection:
// Interface Projection in Repository
List<ParentDtoInterface> findAllInterfaceProjectedBy();
public interface ParentDtoInterface {
long getId();
String getBasic();
List<ChildDtoInterface> getChildren();
}
public interface ChildDtoInterface {
long getId();
}
It results in:
[
{
"id": 1,
"children": [
{
"id": 1
},
{
"id": 2
}
],
"basic": "parent-basic-1"
},
{
"id": 2,
"children": [
{
"id": 3
},
{
"id": 4
}
],
"basic": "parent-basic-2"
}
]
Now my problem with Interface-Projection is, that it will not just load the expected properties, but all properties, but jackson will only serialize those that the Interface provides, cause it uses the Class/Interface-Definition.
Parent loaded: (sql log; see line 4, detail information is loaded)
select
parent0_.id as id1_1_,
parent0_.basic as basic2_1_,
parent0_.detail as detail3_1_
from
parent parent0_
Also Interface Projection seems to be really slow (see this Stackoverflow question) and i still would have to unpack the children cause they are given as [{id:1},{id:2}] but i really need [1,2]. I know i can do this with #JsonIdentityReference(alwaysAsId = true) but thats just a workaround.
Also I'm abit confused why the data is loaded in n+1 queries - 1 for the parents, and another n (where n is the number of parents) for each parents childs:
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
I have tried #OneToMany(fetch=FetchType.LAZY) and #Fetch(FetchType.JOINED) - both give the same result as above.
So the main question is: Is there any way to achive projection with Spring Boot for nested entities, so that only the needed data is loaded in as little as possible queries and in a best case scenario I can adjust it so that instead of having to load List children i can just load List childIds (maybe through a Jpa query that groups the joined rows by parentid and lets be extract needed data from the Child?).
Im using Hibernate and an In-Memory Database.
Thanks in regards for any answer or tip!
Edit: To clarify: I'm not trying to find a way to serialize the data in the wanted format - this i already can achive. The main focus is on only loading the neccessary information from the database.

Answer 1

this will always fetch the children but could give you the result you want.
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);
}
}