public function getValidation($field = null)
    {

        $validation = array();
        $schema = $this->getModelSchema();

        if ($field != null)
            $schema = array($field => $schema[$field]);

        foreach ( $schema as $key => $value ) {
            if (!isset($value['validation']))
                throw new Exception('Validation missing for key: '.$field);
            $validation[$key] = $value['validation'];
        }

        if ($field != null)
            return $validation[$field];

        return $validation;
    }