save not working properlly

save not working properlly

controller
userdetails.php
class Controller_Userdetails extends Controller {
public function action_index() {
$view = new View('userdetails/index');
$this->response->body($view);
}
public function action_add() {//load adddetails.php
$userdetails = new Model_Userdetails();
$view = new View('userdetails/adddetails');
$view->set("userdetails", $userdetails);
$this->response->body($view);
}
public function action_post() {//save the post data
$userdetails_id = $this->request->param('id');
$userdetails = new Model_Userdetails($userdetails_id);
$userdetails->values($_POST);
$userdetails->save();
$this->request->redirect('index.php/userdetails');
}
}
views
adddetails.php
<?php echo Form::open('userdetails/post/'.$userdetails->id); ?>
<?php echo Form::label("first_name", "First Name"); ?>
<?php echo Form::input("first_name", $userdetails->first_name); ?>
<br />
<?php echo Form::label("last_name", "Last Name"); ?>
<?php echo Form::input("last_name", $userdetails->last_name); ?>
<br />
<?php echo Form::label("email", "Email"); ?>
<?php echo Form::input("email", $userdetails->email); ?>
<br />
<?php echo Form::submit("submit", "Submit"); ?>
<?php echo Form::close(); ?>
I am trying to insert the data into database.The name and email field are
loading correctly,if i enter values and hit enter it is redirecting to
other page but objects not saved in database.Need help to solve this.