async function _updateExec(query, op) {
  const options = query._optionsForExec();

  const criteria = query._conditions;
  const doc = query._updateForExec();

  debug('update', query._collection.collectionName, criteria, doc, options);

  return query._collection[op](criteria, doc, options);
}
Helper for de-duplicating "update*" functions
@param {Query} query The Query Object (replacement for "this")
@param {String} op The Operation to be done
@returns the results