Dynamic projections for interfaces does not work

[mipo256]

I've created test in the playground repo to demonstrate the issue.

The fact is - currently, the JdbcQueryCreator.complete() fails to build a SELECT clause in case of interfaces to be used in dynamic projections. The stack trace is:

java.lang.IllegalStateException: SELECT does not declare a select list

	at org.springframework.data.relational.core.sql.SelectValidator.doValidate(SelectValidator.java:57)
	at org.springframework.data.relational.core.sql.SelectValidator.validate(SelectValidator.java:49)
	at org.springframework.data.relational.core.sql.DefaultSelectBuilder.build(DefaultSelectBuilder.java:202)
	at org.springframework.data.jdbc.repository.query.JdbcQueryCreator.complete(JdbcQueryCreator.java:182)
	at org.springframework.data.jdbc.repository.query.JdbcQueryCreator.complete(JdbcQueryCreator.java:66)
	at org.springframework.data.repository.query.parser.AbstractQueryCreator.createQuery(AbstractQueryCreator.java:95)

That seems to happen only for dynamic projections, interface based projection on their own should be fine.

I think it's going to be great to add a support for this.

[ngocnhan-tran1996]

@schauder

This is a feature that will support interface based projection like below code?

interface MyEntityInterfaceProjection {

    String status();

    Long id();
}

[mipo256]

Not, exactly @ngocnhan-tran1996.

This issue target the dynamic projects more specifically, which can use interface of course as the target type. The interface based projections should work, though, as they are documented. You have any issues?

[ngocnhan-tran1996]

@mipo256

correct me if I miss something.

I take a look your testcase and it doesn't work, but if I add get prefix, it works well.
So I'm confused about this part, this will be a feature that no need get prefix?

From document

The easiest way to limit the result of the queries to only the name attributes is by declaring an interface that exposes accessor methods for the properties to be read, as shown in the following example:

    @Test
    @Disabled // Enable to see what happens
    void whenDynamicProjectsAreInUseViaInterfaces_thenBlast() {
        saveInitialEntity();
        Optional<MyEntityInterfaceProjection> active = myEntityRepository.findProjectionByStatus("active", MyEntityInterfaceProjection.class);

        Assertions.assertThat(active)
            .isPresent()
            .hasValueSatisfying(myEntityProjection -> {
               Assertions.assertThat(myEntityProjection.id() != null && myEntityProjection.status().equalsIgnoreCase("active")).isTrue();
            });
    }

   // more code

    interface MyEntityInterfaceProjection {
        String status();  // add get prefix
        Long id(); // add get prefix
    }

[mipo256]

Yeah, it does indeed.
I think it would be great to resolve this as well @ngocnhan-tran1996
The error message itself actually is produced by SelectValidator because it just does not have any fields for selection. At the very least, the message should be corrected.

[schauder]

As @ngocnhan-tran1996 correctly pointed out, the exception happens, because the interface doesn't have any getters.
This won't change.

But I took up your suggestion of a more explicit exception with the referenced PR.

[mp911de]

I don't think catching an exception is the right approach. Instead, if the list of input properties is empty, then we should select everything. We had similar cases with projections that use @Value without adhering to the beans syntax.

In such a case, we expect that the underlying object is fully materialized so that projections can compute their own values.

[mp911de]

FWIW, the same issue exists in JPA. With spring-projects/spring-data-commons#3164, I fixed the ProjectionInformation.isClosed() check. This implicitly fixes ReturnedType.needsCustomConstruction(). Maybe this is already sufficient to address the issue in Spring Data Relational.