public function pointerForRelationship(string $key, string $default = '/'): string
    {
        $field = collect(explode('.', $key))->first();

        if (!$this->isRelationship($field)) {
            throw new \InvalidArgumentException("Field {$field} is not a relationship.");
        }

        $pointer = $this->pointer($key);

        return Str::after($pointer, "relationships/{$field}") ?: $default;
    }