public function addMeta($key, $value)
    {
        $existing = $this->meta()
            ->where('key', $key)
            ->where('value', Helpers::maybeEncode($value))
            ->first();

        if ($existing) {
            return false;
        }

        $meta = $this->meta()->create([
            'key'   => $key,
            'value' => $value,
        ]);

        return $meta->isSaved() ? $meta : $meta->getErrors();
    }