single = (refArray) => {
                if (refArray === null) {
                    // an intermediate field could not be resolved
                    return null;
                }
                return refArray.map((ref) =>
                    this._selector(state, ref[idAttribute])
                );
            }
The parent selector should return a ref array
in case of a single ID being passed.
Otherwise it should return an array of ref arrays.