docs: function (userId, user, fields, modifier) {
      // make sure it is our record
      if (user._id !== userId)
        return false;
  
      // user can only modify the 'profile' field. sets to multiple
      // sub-keys (eg profile.foo and profile.bar) are merged into entry
      // in the fields list.
      if (fields.length !== 1 || fields[0] !== 'profile')
        return false;
  
      return true;
    }