code: function (callback) {
      IdentityCounter.findOne({
        model: settings.model,
        field: settings.field
      }, function (err, counter) {
        if (err) return callback(err);
        callback(null, counter === null ? settings.startAt : counter.count + settings.incrementBy);
      });
    }
docs: Declare a function to get the next counter for the model/schema.