code: public <R, RK> List<R> fetch(Collection<DBRef<R, RK>> collection) {
          return fetch(collection, null);
      }
docs: Fetch a collection of dbrefs.  This is more efficient than fetching one at a time.
  
  @param collection the collection to fetch
  @param <R>        The type of the reference
  @return The collection of referenced objcets