A custom SchemaType for Mongoose that treats `_id` as a string on the mongoose document, but as an ObjectId in the database.

object-id-as-string.js

/*
 * ObjectIdAsString
 *
 * A SchemaType for MongooseJS that acts exactly like an ObjectId except that
 * the field is handled as a string in the mongoose document.
 *
 * NOTE: If a document is fetched with `lean:true` then the field is not converted
 * to a string. Ugh.
 *
 *     //register the type
 *     var mongoose = require('mongoose');
 *     require('./object-id-as-string')(mongoose);
 *
 *     //use it on your schema
 *     var schema = mongoose.Schema({
 *       _id: { type: "ObjectIdAsString" },
 *       ....
 *     });
 *     var Thing = mongoose.model("Thing", schema);
 *
 *     //creating a document
 *     var thing = new Thing();
 *     thing._id; //this is a string like "43345823304969c878318d12"
 *
 *     //saving
 *     thing.save(function(err){
 *       thing._id; //this is still a string
 *       //the database now has { _id: ObjectId("43345823304969c878318d12") }
 *     });
 *
 *     //fetching works like normal
 *     Thing.findOne({ _id: "43345823304969c878318d12" }, function(err, thing){
 *       thing._id; //this is a string
 *     });
 *     Thing.findOne({ _id: { $in: [ObjectId("43345823304969c878318d12")] } }, function(err, thing){
 *       thing._id; //this is a string
 *     });
 *     
 *     //lean doesn't quite work
 *     Thing.findOne({ _id: "43345823304969c878318d12" }).lean().exec(function(err, thing){
 *       thing._id; //this is an ObjectId, not a string
 *     });
 *
 */

var util = require('util');

module.exports = exports = function (mongoose) {

  function ObjectIdAsString (path, options) {
    //call the standard ObjectId constructor
    mongoose.SchemaTypes.ObjectId.call(this, path, options);

    //expose the value as a string
    this.get(function(val){
      return val ? String(val) : val;
    });

    //defaults to new ObjectId
    this.default(function(){
      return mongoose.Types.ObjectId();
    });
  }

  //should act like ObjectId
  util.inherits(ObjectIdAsString, mongoose.SchemaTypes.ObjectId);

  //make sure it hasn't been registered
  if (mongoose.SchemaTypes.ObjectIdAsString)
    throw new Error('A ObjectIdAsString schema type is already registered.');

  //expose it as a SchemaType
  mongoose.SchemaTypes.ObjectIdAsString = ObjectIdAsString;

  //return schema type
  return ObjectIdAsString;
};