使用Eloquent获取联接表上的最新值

Question

我有两个这样的表:

产品:

+----+-----------+
| id |   name    |
+----+-----------+
|  1 | Product 1 |
|  2 | Product 2 |
|  3 | Product 3 |
|  4 | Product 4 |
+----+-----------+

价格:

+----+-------+------------+---------------------+
| id | price | product_id |     created_at      |
+----+-------+------------+---------------------+
|  1 |    20 |          1 | 2014-06-21 16:00:00 |
|  2 |    10 |          1 | 2014-06-21 17:00:00 |
|  3 |    50 |          2 | 2014-06-21 18:00:00 |
|  4 |    40 |          2 | 2014-06-21 19:00:00 |
+----+-------+------------+---------------------+

我在产品上有这种关系:

public function prices()
{
    return $this->hasMany('Price');
}

我可以轻松地运行Product::with('prices')->get();以获得每个产品的每个价格.

我怎样才能使用Eloquent才能获得最新的价格？(另外,如果我想要最便宜/最昂贵的价格呢？)

Answer 1

你可以调整你的关系,以获得你想要的.接受的答案当然有效,但它可能是大量数据的内存过多.

在这里和那里了解更多.

以下是如何使用Eloquent:

// Product model
public function latestPrice()
{
   return $this->hasOne('Price')->latest();
}

// now we're fetching only single row, thus create single object, per product:
$products = Product::with('latestPrice')->get();
$products->first()->latestPrice; // Price model

这很好,但还有更多.想象一下,您想为所有产品加载最高价格(只是一个价值):

public function highestPrice()
{
   return $this->hasOne('Price')
      ->selectRaw('product_id, max(price) as aggregate')
      ->groupBy('product_id');
}

不太方便:

$products = Product::with('highestPrice')->get();
$products->first()->highestPrice; // Price model, but only with 2 properties
$products->first()->highestPrice->aggregate; // highest price we need

因此,添加此访问器可以让生活更轻松:

public function getHighestPriceAttribute()
{
    if ( ! array_key_exists('highestPrice', $this->relations)) $this->load('highestPrice');

    $related = $this->getRelation('highestPrice');

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

// now it's getting pretty simple
$products->first()->highestPrice; // highest price value we need