Spring Boot 和 JPA：将数据从多连接查询返回到 GET HTTP 命令时出错

Question

I'm on Spring Boot and JPA since few weeks, so maybe the question is relatively trivial to someone. I have an @Entity where I defined a @NamedNativeQuery like:

@NamedNativeQuery(
        name = "Device.getMeasuresAndDates",
        query = """
        select a.id, 
        a.device,
        a.deviceManufacturer, 
        b.dateOfMeasure, 
        c.Id, 
        c.componentSerialNumber, 
        c.ComponentMeasure
        from devices a inner join measures b on b.device_id = a.id
        inner join instruments c on c.id = b.instrument_id
        where a.device = :device
        order by b.dateOfMeasure, c.componentSerialNumber
        """,
        resultSetMapping = "MeasuresAndDatesMapping"
        )

the query is defined in the Device entity, as it seems to be the logical place where to put it. The query returns only proper fields, not all the entities fields that are much more, and most of them are useless to the service.

then I tried to map the results with @SqlResultSetMapping + @EntityResult; since I do not return all table fields, I do not map all entity properties, I put the entity property names in "name" attribute and the field returned by the query in "column" attribute:

@SqlResultSetMapping(
        name = "MeasuresAndDatesMapping",
        entities = {
            @EntityResult(
                    entityClass = Device.class,
                    fields = {
                        @FieldResult(name = "id", column = "a.id"),
                        @FieldResult(name = "device", column = "a.device"),
                        @FieldResult(name = "deviceManufacturer", column = "a.deviceManufacturer")
                    }
            ),
            @EntityResult(
                entityClass = Measure.class,
                fields = {
                    @FieldResult(name = "dateOfMeasure", column = "b.dateOfMeasure")
                }
            ),
            @EntityResult(
                    entityClass = Component.class,
                    fields = {
                        @FieldResult(name = "id", column = "c.id"),
                        @FieldResult(name = "componentSerialNumber", column = "c.componentSerialNumber"),
                        @FieldResult(name = "componentMeasure", column = "c.ComponentMeasure")
                    }
            )
        }
    )

then I mapped the query to the JPA repository defined for Device:

@Query(nativeQuery = true, name="Device.getMeasuresAndDates") 
public List<Object> LetturePerPod(@Param("device") String device);

First, I'm not sure what type has to be returned by the interface method. Second, I do not know if it's correct to define such a method in the interface. But if I want to define the method directly in the entity class, I have to access the EntityManager to use the @NamedNativeQuery, but that is not possible from within the class. I aim to keep the architecture as clean as possible, leveraging JPA and avoiding any tricky solutions.

Anyway, doing things this way I get this error:

Cannot createTypedQuery for query with more than one return

I guess it would work only if I return all fields of all entities, like

select
a.*,
b.*,
c.*
from

so which is the right approach to the problem?

Any suggestions are appreciated

Answer 1

It looks like that it's your column "id" which is duplicated in a and c. Just like in sql you could just add an alias to the duplicated columns.

@NamedNativeQuery(
    name = "Device.getMeasuresAndDates",
    query = """
    select a.id as aid, 
    a.device,
    a.deviceManufacturer, 
    b.dateOfMeasure, 
    c.Id as cid, 
    c.componentSerialNumber, 
    c.ComponentMeasure
    from devices a inner join measures b on b.device_id = a.id
    inner join instruments c on c.id = b.instrument_id
    where a.device = :device
    order by b.dateOfMeasure, c.componentSerialNumber
    """,
    resultSetMapping = "MeasuresAndDatesMapping"
    )

PS: SQL doesn't care about uppercase Id and ID and id is the exact same field.