Modify SQL query generated behind Spring Data REST

Question

EDIT : How to Keep only needed columns in SELECT for Spring Data Rest Projections?

Spring Data Rest Projections are good for getting a subset of columns for links which are generated, but the Query that gets generated in behind still has all columns in it.

How can Projections be created where also SQL queries have only those columns in SELECT which are in Projection

Answer 1

I think you can use "nested projections", let's say.

An example: resource A contains field bubi and resource B, which has field foo, bar, zed.

You should create a projection for B where you list just the fields you want:

@Projection(name="reduced", types = B.class)
public interface BReduced {
    String foo;
    //exclude bar, for instance
    int zed;
}

Then you use that projection in A's projection.

@Projection(name="reduced", types = A.class)
public interface AReduced {
    int bubi;
    BReduced b;
}

Have I undestood well, or were you talking about performance in SQL query?

Answer 2

I don't know why it's missing from the docs, but this spring sample (from spring) shows that you can use projections as the return type for a @Query. So you can do:

public interface ActionId {
    String getId(); 
}

@Query("select a.id as id from Action a where a.type = :type")
public List<ActionId> findByType(@Param("type") String type);

Now instead of having to use constructor expressions, you can more succinctly just select the columns you want, and return an object. I wish that projections could be applied to the domain object itself, so you could still return an "Action" with just the id field, but that doesn't look possible now-

ref:https://github.com/spring-projects/spring-data-examples/blob/master/jpa/example/src/main/java/example/springdata/jpa/projections/CustomerRepository.java