雄辩的“select”方法不能与使用“with”方法一起使用

Question

我的村庄模式；

<?php
namespace App;
use Illuminate\Database\Eloquent\Model;

class Village extends Model {
    public function positions() {
        return $this->belongsTo(Map::class, 'id', 'field_id');
    }
}

我的地图类迁移；

Schema::create('map_data', function (Blueprint $table) {
    $table->increments('id');
    $table->integer('field_type');
    $table->integer('field_id');
    $table->string('x');
    $table->string('y');
    $table->timestamps();
});

public function villages() {
    $villages = Village::with([
        'positions' => function ($query) {
            $query->select('x', 'y');
        }
    ])->get();

    return $villages;
}

{
  "villages": [
    {
      "id": 1,
      "name": "village 1",
      "created_at": "2016-10-26 18:36:34",
      "updated_at": "2016-10-26 18:36:34",
      "positions": null
    },
    {
      "id": 2,
      "name": "village 2",
      "created_at": "2016-10-26 18:36:34",
      "updated_at": "2016-10-26 18:36:34",
      "positions": null
    }
  ]
}

{
  "villages": [
    {
      "id": 1,
      "name": "village 1",
      "created_at": "2016-10-26 18:36:34",
      "updated_at": "2016-10-26 18:36:34",
      "positions": {
        "id": 1,
        "field_type": "1",
        "field_id": "1",
        "x": "21",
        "y": "21",
        "created_at": "2016-10-26 18:36:34",
        "updated_at": "2016-10-26 18:36:34"
      }
    },
    {
      "id": 2,
      "name": "village 2",
      "created_at": "2016-10-26 18:36:34",
      "updated_at": "2016-10-26 18:36:34",
      "positions": {
        "id": 2,
        "field_type": "1",
        "field_id": "2",
        "x": "0",
        "y": "0",
        "created_at": "2016-10-26 18:36:34",
        "updated_at": "2016-10-26 18:36:34"
      }
    }
  ]
}

Answer 1

当你试图急切地为村庄加载位置关系时

Village::with(['positions'])->get();

发生了3件事：

1. 雄辩负载所有村庄
2. 雄辩加载所有位置
3. ELOQUENT使用field_id列为相应的村庄对象分配位置

为了使其工作，提取的位置需要提取field_id列，否则Eloquent无法将其与相应的村庄匹配。

当你这么做的时候

$query->select('x', 'y');

您只能从positions表中获取x和y列。field_id列没有被提取，这就是为什么Eloquent不能用Village对象提取它们，这就是为什么您在其中输入null而不是一个位置集合。

$query->select('x', 'y');

$query->select('field_id', 'x', 'y');

以使代码按预期工作。