Below is a typical validations rules for User Model. The validations rules of a field are ran from bottom up. In this case, for the email field, it will run stringLength, email and then isUnique rules.
<?php class User extends AppModel { var $name = "User"; // Validation rules. var $validate = array( 'username'=>array( 'isUnique'=>array( 'rule'=>'isUnique', 'message'=>'This username has already been taken.', ), 'stringLength'=>array( 'rule'=>array('between', 1, 40), 'message'=>'This field must be between 1 and 40 characters long.', ), ), 'password'=>array( 'stringLength'=>array( 'rule'=>array('between', 6, 40), 'message'=>'This field must be between 6 and 40 characters long.', ), ), 'email'=>array( 'isUnique'=>array( 'rule'=>'isUnique', 'message'=>'This email already existed.', ), 'email'=>array( 'rule'=>array('email', true), 'message'=>'Please supply a valid email address.', ), 'stringLength'=>array( 'rule'=>array('between', 1, 40), 'message'=>'This field must be between 1 and 40 characters long.', ), ), ); } ?>