code: public DBRef keyToDBRef(final Key key) {
          if (key == null) {
              return null;
          }
          if (key.getType() == null && key.getCollection() == null) {
              throw new IllegalStateException("How can it be missing both?");
          }
          if (key.getCollection() == null) {
              key.setCollection(getCollectionName(key.getType()));
          }
  
          Object id = key.getId();
          if (isMapped(id.getClass())) {
              id = toMongoObject(id, true);
          }
          return new DBRef(key.getCollection(), id);
      }
docs: Converts a Key to a DBRef
  
  @param key the Key to convert
  @return the DBRef