通过级联删除父项目时，删除Laravel 5.3多态项目

use Illuminate\Database\Eloquent\Model;

class ParentModel extends Model
{
    public function child()
    {
        return $this->morphMany(ChildModel::class, 'parent');
    }
}

use Illuminate\Database\Eloquent\Model;

class ChildModel extends Model
{
    public function parent()
    {
        return $this->morphTo();
    }
}

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;

class CreateParentModelsTable extends Migration
{
    public function up()
    {
        Schema::create('parent_models', function (Blueprint $table) {
            $table->id();
            // 定义其他字段
            $table->timestamps();
        });

        Schema::create('child_models', function (Blueprint $table) {
            $table->id();
            $table->unsignedBigInteger('parent_id');
            $table->string('parent_type');
            // 定义其他字段
            $table->timestamps();

            $table->foreign('parent_id')->references('id')->on('parent_models')->onDelete('cascade');
        });
    }

    public function down()
    {
        Schema::dropIfExists('child_models');
        Schema::dropIfExists('parent_models');
    }
}

use App\Models\ParentModel;

class ParentController extends Controller
{
    public function delete($id)
    {
        $parent = ParentModel::findOrFail($id);
        $parent->delete();

        // 删除父项目时，与之关联的子项目也会被自动删除
    }
}