The Controller file at app/controllers/post_controller.php

class PostController extends ApplicationController {
 
  function create(){
    $this->Post =& new Post($this->params["post"]);
    if($this->Post->save()){
      $this->redirectToAction("show", array('id' => $Post->getId()));
    }else{
      $this->Post->errors = $this->Post->getErrors();
    }
  }
 
  //...
  function show(){
  }
 
}

The View file at app/views/post/show.tpl

{?Post.errors}
  The post couldn't be saved due to these errors:
  <ul>
  {loop Post.errors}
    <li>{error}</li>
  {end}
  </ul>
{end}
 
<%= form "post" %>

or even shorter:

<%= error_messages_for 'post' %>

The Model file at app/models/post.php

class Post extends ActiveRecord
{
    function validate(){
      $this->validatesPresenceOf( array('title', 'body'), "Missing required field");
      $this->get('title') > 10 ? $this->addError("title", "must be at most 10 characters") : null;
    }
}

• Understanding Validations
• Validation API Documentation has all sorts of details about what methods are available.

Note: When using validatesConfirmationOf() make sure you also use validatesPresenceOf() if you want to make sure the field exists.