MongoDB in Codeigniter - Model

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class Users extends CI_Model {

    function __construct() {
        parent::__construct();
        $tableName = 'users';
        $primaryKey = 'id';
        // Connect to Mongo
        $this->connection = new Mongo('localhost:27017');
        // Select a database
        $this->db = $this->connection->blog;
        // Select a collection
        $this->posts = $this->db->$tableName;
    }

    /** Insert new record */
    function save($member='') {
        if ($member != ''){
            if (!isset($member['id'])){ // new record
                $this->posts->insert($member);
                return $member['_id'];
            } else { // edit existing record
                $memberid = $member['id'];
                $this->posts->update(array('_id' => new MongoId($memberid)), $member, array("multiple" => false));
                return $memberid;
            }
        }
    }

    /** Fetches all records with limit and orderby values's */
    function getAll($limit='', $orderby='') {
        $members = $this->posts->find();
        if ($limit != ''){ $members->limit($limit);}
        if ($orderby != ''){$members = $members->sort($orderby);}
        return $members;
    }

    /** Fetches a record by its' passed field and values's */
    function getByID($id='') {
        $member = $this->posts->findOne(array('_id' => new MongoId($id)));
        if ($member) {
            return $member;
        }
        return false;
    }

    /** Fetches a record by its' passed field and values's */
    function getByColumn($field='id', $value='') {
        $member = $this->posts->findOne(array($field => $value));
        if ($member) {
            return $member;
        }
        return false;
    }

    /** Deletes a record by it's primary key */
    function deleteById($id) {
        $this->posts->remove(array('_id' => new MongoId($id)), array("justOne" => true));
    }

}

hi
how i can select particular fields using mongodb and codeigniter

similarly like "select name from user where user_id =1"