build find and count queries with MongoRepository when dao use DBRef with Map

Question

Whats the name of "count" and "find" queries when i use DBRef in conjunction with java.util.map?

In my example the countByFoos-Method always returns 0 instead of 1.

How can i find and count BarDaos by given FooDao?

public class FooDao {

   @Id
   private String id;

   private String key; 

   ...
}

public class BarDao {

   @Id
   private String id;

   @DBRef
   private Map<String, FooDao> foos;

   ...
}

public interface FooRepository extends MongoRepository<FooDao, String> {
   ...
}

public interface BarRepository extends MongoRepository<BarDao, String> {

   long countByFoos(FooDao fooDao);

   ...
}

public void doSomthing() {
   FooDao foo = new FooDao("fooId1", "key1");
   fooRepository.save(foo);

   BarDao bar = new BarDao("barId1");
   bar.addFoo(foo.getKey(), foo);
   barRepository.save(bar);

   int count = barRepository.countByFoos(foo);
}