如何处理具有实体关系的Spring Boot / Spring Data投影（嵌套投影）

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.

1. 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

2. 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.

3. 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);
    }
}