gistfile1.js

var MongooseUpserts = function (schema, options) {
    options = options || {};

    schema.method('removeDefaults', function () {
        var paths = schema.paths;
        var defaults = {};
        for (var key in paths) {
            if (this._doc[key] === paths[key].defaultValue) {
                delete this._doc[key];
            }
        }
        return this; 
    });
    
    schema.method('upsert', function (conditions, update, callback) {
        var Model = this.constructor;
        var oid = Mongoose.Types.ObjectId;
        var defaultConditions = {_id: this._doc._id };
        var opts = options.options || {upsert: true};
        
        delete this._doc._id;
        var newDoc = this.removeDefaults().toObject();
        if (typeof conditions === 'undefined') { 
            Model.update(defaultConditions, {$set: newDoc}, opts, function (e) {
                console.log(e);
            });
        } else if (typeof conditions === 'function') { 
            Model.update(defaultConditions, {$set: newDoc}, opts, conditions);
        } else if (typeof update === 'function') {
            if (typeof conditions === 'string') { 
                Model.update(defaultConditions, {$set: newDoc}, opts, update);
            } else if (/\$/.test(Object.keys(conditions).toString())) {
                Model.update(defaultConditions, conditions, opts, update);
            } else { 
                Model.update(conditions, {$set: newDoc}, opts, update);
            }
        } else if (typeof callback === 'function') { 
            Model.update(conditions, update, opts, callback);
        } else {
            if (typeof conditions === 'string') { 
                Model.update(oid.fromString(conditions), {$set: newDoc}, opts, function (e) {
                    console.log(e);
                });
            } else {
                throw new Error('Invalid parameters for upsert method.');
            }  
        }
    });
};

Mongoose.plugin(MongooseUpserts, {debug:true, options: { multi: true, upsert: true }});