How to fetch a field from document using n1ql with spring-data-couchbase

Question

This is the query in the repository which works fine.

public interface PlayerRepo extends CrudRepository<Player, String>{

@Query("#{#n1ql.selectEntity} WHERE code = $1 and #{#n1ql.filter}")
public List<Player> findPlayersWithCode(String code);
}

I want to fetch specific field from Player document, like this:

@Query("#{#n1ql.selectEntity.name} WHERE code = $1 and #{#n1ql.filter}")
public List<String> findPlayerNamesWithCode(String code);

Is it possible to do this, I am using spring-data-couchbase 2.1.2

Answer 1

Currently, this is not possible from a repository. Spring Data Hopper introduced a Projection feature that would allow it, but the Couchbase store implementation doesn't yet cover it.

There's something close in the CouchbaseTemplate, which has a findByN1QLProjection method, but it needs a DTO dedicated to the query. For example:

SELECT name, lastName FROM #{#n1ql.bucket} WHERE code = $1 AND #{#n1ql.filter}

which is equivalent (in pure N1QL) to:

SELECT name, lastName FROM bucketThatBacksRepository WHERE code = $1 AND _class = "com.example.Person"

would require the following class to work with findByN1QLProjection:

public class PersonNameDTO {
    private final String name;
    private final String lastName;

    //constructor and maybe getters/setters needed but omitted here
}

And it would produce a List<PersonNameDTO>. Note that this is not hugely different from the projection feature I was talking about, except it's designed around interfaces rather than concrete DTO classes.

Note that you shouldn't use the #{#n1ql.selectEntity} SpEL since its purpose is to have a large SELECT clause that covers all the fields of a given entity (whereas here you want to restrict the SELECT clause).

It also covers the FROM part with the correct bucket associated to your repository, but the #{#n1ql.bucket} also covers that part (and that part only)...