private function extractRelationship(array $parts)
    {
        if (isset($parts[2])) {
            if ('relationships' === $parts[2]) {
                if (!isset($parts[3])) {
                    throw RestException::invalidRelationshipEndpoint($this->parsedUri['path']);
                }
                $this->relationship = [
                    'type'  => 'self',
                    'field' => $parts[3],
                ];
            } else {
                $this->relationship = [
                    'type'  => 'related',
                    'field' => $parts[2],
                ];
            }
        }
        return $this;
    }