在 Laravel 8.0 使用 ORM 實現外鍵的自我參考

Laravel
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前言

本文以一個使用者可以有多項任務（Task），一項任務可以有多項子任務為例。

做法

新增 Task 模型的資料表：

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use App\Models\Group;
use App\Models\User;
use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;

class CreateTasksTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('tasks', function (Blueprint $table) {
            $table->id();
            $table->string('name');
            $table->foreignIdFor(Group::class)->nullable()->constrained()->onDelete('cascade');
            $table->foreignIdFor(User::class)->constrained()->onDelete('cascade');
            $table->foreignId('parent_id')->nullable()->constrained('tasks')->onDelete('cascade');
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('tasks');
    }
}

修改 User 模型：

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namespace App\Models;

use Illuminate\Contracts\Auth\MustVerifyEmail;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Foundation\Auth\User as Authenticatable;
use Illuminate\Notifications\Notifiable;
use Laravel\Sanctum\HasApiTokens;

class User extends Authenticatable
{
    use HasApiTokens, HasFactory, Notifiable;

    // ...

    // 找出隸屬於使用者的任務
    public function tasks()
    {
        return $this->hasMany(Task::class)->whereNull('parent_id');
    }
}

新增 Task 模型：

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namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class Task extends Model
{
    use HasFactory;

    // 找出隸屬於任務的子任務
    public function tasks()
    {
        return $this->hasMany(Task::class, 'parent_id');
    }
}

找出使用者所有的任務，以及任務的子任務：

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User::query()->with([
    'tasks.tasks',
])->first();