public function getByKey($key, $locale, $group = null) {
        $query = $this->source
            ->select('languages.slug as locale', 'translations.*')
            ->join('languages', 'languages.id', '=','translations.language_id')
            ->where('languages.slug', $locale)
            ->whereKey($key);

        if($group)
            $query->where('group', $group);

        return $query
            ->first();
    }