Put ValidationMessageBehavior.php inside app/Model/Behavior folder

Attach behavior in your CakePHP Model

public $actsAs = array('ValidationMessage');

Define Validation Rule

public $validate = array(
	'slug' => array(
		'notempty',
		'isUnique'
	),
	'content' => array(
		'alphanumer',
	),
);

A validation message will automatically be attached to the rule, you don't have to explicitly specify the message;

Supports CakePHP 2.xxx versions.