protected function validateField($field = null)
    {
        $field = $field ?: post(self::PARAM_FIELD);

        if ($field && $field != $this->field) {
            $this->initRelation($this->model, $field);
        }

        if (!$field && !$this->field) {
            throw new ApplicationException(Lang::get('backend::lang.relation.missing_definition', compact('field')));
        }

        return $field ?: $this->field;
    }