Laravel中多对多关系编辑时如何预选下拉列表项

// app/Models/Student.php
namespace App\Models;

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

class Student extends Model
{
    use HasFactory;

    public function appliances()
    {
        // 假设中间表名为 'dealer_appliances'
        return $this->belongsToMany(Appliance::class, 'dealer_appliances');
    }

    public function phones()
    {
        return $this->hasMany(Phone::class);
    }
}

// app/Models/Appliance.php
namespace App\Models;

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

class Appliance extends Model
{
    use HasFactory;

    public function students()
    {
        return $this->belongsToMany(Student::class, 'dealer_appliances');
    }
}

// app/Http/Controllers/StudentController.php
namespace App\Http\Controllers;

use App\Models\Student;
use App\Models\Appliance;
use Illuminate\Http\Request;

class StudentController extends Controller
{
    public function edit($id)
    {
        // 1. 获取指定学生及其已关联的电器
        // 'with('appliances')' 会加载学生关联的电器，避免N+1查询问题
        $student = Student::with('appliances')->findOrFail($id);

        // 2. 获取所有可用的电器列表
        // 通常我们只需要ID和名称来构建下拉列表
        $allAppliances = Appliance::all(['id', 'name']);

        return view('students.edit', compact('student', 'allAppliances'));
    }

    // ... 其他方法，如 update
}

<!-- resources/views/students/edit.blade.php -->

<form action="{{ route('students.update', $student->id) }}" method="POST">
    @csrf
    @method('PUT')

    <label for="appliances">选择电器:</label>
    <select name="appliances[]" id="appliances" multiple class="form-control">
        @foreach($allAppliances as $appliance)
            <option value="{{ $appliance->id }}"
                {{ in_array($appliance->id, $student->appliances->pluck('id')->toArray()) ? 'selected' : '' }}>
                {{ $appliance->name }}
            </option>
        @endforeach
    </select>

    <button type="submit" class="btn btn-primary mt-3">更新学生信息</button>
</form>

// app/Http/Controllers/StudentController.php

class StudentController extends Controller
{
    // ... edit 方法

    public function update(Request $request, $id)
    {
        $student = Student::findOrFail($id);

        // 验证输入
        $validatedData = $request->validate([
            // ... 其他学生字段的验证规则
            'appliances' => 'nullable|array', // 确保 appliances 是一个数组
            'appliances.*' => 'exists:appliances,id', // 确保数组中的每个ID都存在于 appliances 表中
        ]);

        // 更新学生自身信息
        // $student->update($request->only([...]));

        // 使用 sync() 方法更新多对多关系
        // sync() 会自动添加、删除或保留关联，以匹配传入的ID数组
        $student->appliances()->sync($request->input('appliances', []));

        return redirect()->route('students.edit', $student->id)->with('success', '学生信息及电器已更新！');
    }
}