index.js

// Module Scope
var mongoose = require('mongoose'),
extend = require('extend'),
counterSchema,
IdentityCounter;

// Initialize plugin by creating counter collection in database.
exports.initialize = function (connection) {
  try {
    IdentityCounter = connection.model('IdentityCounter');
  } catch (ex) {
    if (ex.name === 'MissingSchemaError') {
      // Create new counter schema.
      counterSchema = new mongoose.Schema({
        model: { type: String, require: true },
        field: { type: String, require: true },
        count: { type: Number, default: 0 }
      });

      // Create a unique index using the "field" and "model" fields.
      counterSchema.index({ field: 1, model: 1 }, { unique: true, required: true, index: -1 });

      // Create model using new schema.
      IdentityCounter = connection.model('IdentityCounter', counterSchema);
    }
    else
      throw ex;
  }
};

// The function to use when invoking the plugin on a custom schema.
exports.plugin = function (schema, options) {

  // If we don't have reference to the counterSchema or the IdentityCounter model then the plugin was most likely not
  // initialized properly so throw an error.
  if (!counterSchema || !IdentityCounter) throw new Error("mongoose-auto-increment has not been initialized");

  // Default settings and plugin scope variables.
  var settings = {
    model: null, // The model to configure the plugin for.
    field: '_id', // The field the plugin should track.
    startAt: 0, // The number the count should start at.
    incrementBy: 1, // The number by which to increment the count each time.
    unique: true
  },
  fields = {}, // A hash of fields to add properties to in Mongoose.
  ready = false; // True if the counter collection has been updated and the document is ready to be saved.

  switch (typeof(options)) {
    // If string, the user chose to pass in just the model name.
    case 'string':
      settings.model = options;
    break;
    // If object, the user passed in a hash of options.
    case 'object':
      extend(settings, options);
    break;
  }

  // Add properties for field in schema.
  fields[settings.field] = {
    type: Number,
    require: true
  };
  schema.add(fields);

  // Find the counter for this model and the relevant field.
  IdentityCounter.findOne(
    { model: settings.model, field: settings.field },
    function (err, counter) {
      if (!counter) {
        // If no counter exists then create one and save it.
        counter = new IdentityCounter({ model: settings.model, field: settings.field, count: settings.startAt - settings.incrementBy });
        counter.save(function () {
          ready = true;
        });
      }
      else {
        ready = true;
      }
    }
  );

  // Declare a function to get the next counter for the model/schema.
  var nextCount = 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);
    });
  };
  // Add nextCount as both a method on documents and a static on the schema for convenience.
  schema.method('nextCount', nextCount);
  schema.static('nextCount', nextCount);

  // Declare a function to reset counter at the start value - increment value.
  var resetCount = function (callback) {
    IdentityCounter.findOneAndUpdate(
      { model: settings.model, field: settings.field },
      { count: settings.startAt - settings.incrementBy },
      { new: true }, // new: true specifies that the callback should get the updated counter.
      function (err) {
        if (err) return callback(err);
        callback(null, settings.startAt);
      }
    );
  };
  // Add resetCount as both a method on documents and a static on the schema for convenience.
  schema.method('resetCount', resetCount);
  schema.static('resetCount', resetCount);

  // Every time documents in this schema are saved, run this logic.
  schema.pre('save', function (next) {
    // Get reference to the document being saved.
    var doc = this;

    // If the document already has the field we're interested in and that field is a number then run this logic
    // to give the document a number ID that is incremented by the amount specified in relation to whatever the
    // last generated document ID was.
    if (typeof(doc[settings.field]) !== 'number') {
      // Declare self-invoking save function.
      (function save() {
        // If ready, run increment logic.
        // Note: ready is true when an existing counter collection is found or after it is created for the
        // first time.
        if (ready) {
          // Find the counter collection entry for this model and field and update it.
          IdentityCounter.findOneAndUpdate(
            // IdentityCounter documents are identified by the model and field that the plugin was invoked for.
            { model: settings.model, field: settings.field },
            // Increment the count by `incrementBy`.
            { $inc: { count: settings.incrementBy } },
            // new:true specifies that the callback should get the counter AFTER it is updated (incremented).
            { new: true },
            // Receive the updated counter.
            function (err, updatedIdentityCounter) {
              if (err) return next(err);
              // If there are no errors then go ahead and set the document's field to the current count.
              doc[settings.field] = updatedIdentityCounter.count;
              // Continue with default document save functionality.
              next();
            }
          );
        }
        // If not ready then set a 5 millisecond timer and try to save again. It will keep doing this until
        // the counter collection is ready.
        else
          setTimeout(save, 5);
      })();
    }
    // If the document does not have the field we're interested in or that field isn't a number AND the user did
    // not specify that we should increment on updates, then just continue the save without any increment logic.
    else
      next();
  });
};