code: public QueryBuilder put(final String key) {
          _currentKey = key;
          if (_query.get(key) == null) {
              _query.put(_currentKey, new NullObject());
          }
          return this;
      }
docs: Adds a new key to the query if not present yet. Sets this key as the current key.
  
  @param key MongoDB document key
  @return {@code this}