Laravel:jak uzyskać średnią zagnieżdżonych relacji hasMany (hasManyThrough)

Potrzebujesz czegoś takiego http://softonsofa.com/tweaking-eloquent-relations-how-to-get-hasmany-relation-count-effly/ tylko nieznacznie dostosowane do Twoich potrzeb:

public function reviewRows()
{
    return $this->hasManyThrough('ReviewRow', 'Review');
}

public function avgRating()
{
    return $this->reviewRows()
      ->selectRaw('avg(rating) as aggregate, product_id')
      ->groupBy('product_id');
}

public function getAvgRatingAttribute()
{
    if ( ! array_key_exists('avgRating', $this->relations)) {
       $this->load('avgRating');
    }

    $relation = $this->getRelation('avgRating')->first();

    return ($relation) ? $relation->aggregate : null;
}

Następnie tak proste, jak to:

// eager loading
$products = Product::with('avgRating')->get();
$products->first()->avgRating; // '82.200' | null

// lazy loading via dynamic property
$product = Product::first()
$product->avgRating; // '82.200' | null