public function keys($kind, array $options = [])
    {
        $options += [
            'number' => 1,
            'ancestors' => [],
            'id' => null,
            'name' => null
        ];

        if ($options['number'] < 1) {
            throw new \InvalidArgumentException('Number of keys cannot be less than 1.');
        }

        $path = [];
        if (count($options['ancestors']) > 0) {
            $path = $options['ancestors'];
        }

        $path[] = array_filter([
            'kind' => $kind,
            'id' => $options['id'],
            'name' => $options['name']
        ]);

        $key = new Key($this->projectId, [
            'path' => $path,
            'namespaceId' => $this->namespaceId
        ]);

        $keys = [$key];

        for ($i = 1; $i < $options['number']; $i++) {
            $keys[] = clone $key;
        }

        return $keys;
    }