private function filter()
    {
        if ($this->allowsFilter()) {
            if ($this->hasFilters()) {
                $tmp = [];
                foreach ($this->filters as $filter) {
                    // check, if it is a "forbidden" query parameter
                    if ($this->isExcludedParameter($filter['key'])) {
                        continue;
                    }
                    $tmp[] = $filter;
                }

                $this->filters = $tmp;

                return $this->operator->filter($this->filters);
            }
        }
    }