var mongoose = require('mongoose')
      , Schema = mongoose.Schema

  , ObjectId = Schema.ObjectId;

var user_schema = new Schema({
    _id: ObjectId,
    created_at: { type: Date, default: Date.now },
    user_id: Number,
    name: String,
    user_name: String,
    tweet_count: Number,
    friends_count: Number,
    followers_count: Number,
    time_zone: String,
    last_tweet:{}
});

var User = mongoose.model('Users', user_schema);

var Schema = mongoose.Schema,
    ObjectID = Schema.ObjectId;

    var newUser = new User({
                _id: ObjectID,
                created_at: user_rest_data.status.created_at,
                user_id: user_rest_data.id,
                name: user_rest_data.name,
                user_name: user_rest_data.screen_name,
                tweet_count: user_rest_data.statuses_count,
                friends_count: user_rest_data.friends_count,
                followers_count:user_rest_data.followers_count,
                time_zone: user_rest_data.time_zone,
                last_tweet: user_rest_data.status});

            newUser.save(function(err){
                if(err) console.log(util.inspect(err));
                else
                    console.log("New document has been added!");
            });

{ [ValidationError: Users validation failed]
  message: 'Users validation failed',
  name: 'ValidationError',
  errors:
   { _id:
      { [CastError: Cast to ObjectID failed for value "function ObjectId(key, options) {
        SchemaType.call(this, key, options, 'ObjectID');
      }" at path "_id"]
        message: 'Cast to ObjectID failed for value "function ObjectId(key, options) {n  SchemaType.call(this, key, options, 'ObjectID');n}" at path "_id"',
        name: 'CastError',
        kind: 'ObjectID',
        value: [Object],
        path: '_id' } } }