How to add Validation Rules to Propel Models after Model Generation?

How would I add validation rules to my columns after Models have been generated. I know you can extend propel's validation, but I didn't see an option for applying these validation rules to my model's columns after I have generated these models from the schema.

To clarify, I know you add these rules in the actual schema, but I would prefer to not do that.

in any propel generated main class (e.g. User) you can override doValidate($column=null) method. See the example below...the return logic part may require improvements.

public function doValidate($columns = null){
    $parentErrors = parent::doValidate($columns);
    $validationErrors = array();
    if(mb_strlen($this->getFirstName()) == 0){
        $message = '...';
        $vf = new ValidationFailed(UserPeer::FIRST_NAME, $message);
        $validationErrors[] = $vf;
    }



    if(is_array($parentErrors)){
        if(!empty($validationErrors)){
            $validationErrors = array_merge($parentErrors, $validationErrors);
        }else{
            $validationErrors = $parentErrors;
        }
        return $validationErrors;
    }elseif($parentErrors === true){
        if(empty($validationErrors)){
            return true;
        }else{
            return $validationErrors;
        }
    }else{
        throw UnexpectedValueException('Unexpected validation state.');
    }
}

Afterwards you can run $validationErrors = $user->validate(); anywhere in your application and check the return value.

I hope this answers your question.