Laravel 8-由于违反完整性约束，可以删除但不能更新父行：外键约束失败

我有一个卡车桌，一个包裹桌和一个邮递员桌。卡车表和包裹表具有一对多的关系。卡车表和邮递员表具有一对一的关系。

现在我可以删除卡车表了，但是当我尝试更新 postman_name分配了包裹的卡车数据时，我在卡车表中遇到了违反完整性约束的错误。当我尝试更改其他非外键属性或尝试更改postman_name当前分配了零包装的卡车数据时，我没有收到错误消息。

当我尝试，我只得到了错误的更新 postman_name上truck_number与分配给它的包。

如何解决此错误？

里面的邮递员表格truck/edit.blade.php：

<div class="form-group">
                <label for="postman_name">Postman in Charge</label>
                <select name="truck_number" class="form-control">
                <option selected disabled>-</option>
                @foreach ($postmen as $count => $postman)
                  <option value="{{$postman['postman_name']}}">{{$postman['postman_name']}}</option>
                @endforeach
                </select>
</div>

卡车更新控制器：

public function update(Request $request, $truck_id)
    {
        $request->validate([
            'truck_number'=>'required|unique:trucks,truck_id',
            'postman_name',
            'date_of_operation'=>'required'
        ]);
        $trucks = Truck::find($truck_id);
        $trucks->truck_number =  $request->get('truck_number');
        $trucks->postman_name = $request->get('postman_name');
        $trucks->date_of_operation = $request->get('date_of_operation');
        $trucks->save();
        return redirect(TRUCK)->with('success', 'Truck Updated!');
    }

包迁移文件：

Schema::create('packages', function (Blueprint $table) {
            $table->increments('package_id');
            $table->string('truck_number')->nullable(); 
            $table->foreign('truck_number')->references('truck_number')->on('trucks')->onDelete('cascade');
            $table->string('package_number')->unique();
            $table->string('receiver_name');
            $table->string('destination');
            $table->timestamps();
        });

邮递员迁移文件：

Schema::create('postmen', function (Blueprint $table) {
            $table->increments('postman_id');
            $table->string('truck_number')->nullable();
            $table->foreign('truck_number')->references('truck_number')->on('trucks')->onDelete('cascade');
            $table->string('postman_number')->unique();
            $table->string('postman_name');
            $table->timestamps();
        });

卡车迁移文件：

Schema::create('trucks', function (Blueprint $table) {
            $table->increments('truck_id');
            $table->string('truck_number')->unique();
            $table->string('postman_name')->nullable();
            $table->date('date_of_operation');
            $table->timestamps();
        });

楷模：

class postman extends Model
{

    use HasFactory;
    protected $primaryKey = 'postman_id';

    protected $fillable = ['truck_number','postman_number','postman_name'];

    public function Truck(){
        return $this->belongsTo(Truck::class);
    }
}

class Package extends Model
{
    use HasFactory;
    protected $primaryKey = 'package_id';

    protected $fillable = ['truck_number', 'package_number', 'receiver_name', 'destination'];
    
     public function Truck(){
        return $this->belongsTo(Truck::class);
    }

}

class Truck extends Model
{
    use HasFactory;
    protected $primaryKey = 'truck_id';
    protected $fillable = ['truck_number', 'postman_name', 'date_of_operation', 'status'];
    
     public function Package()
    {
        return $this->hasMany(Package::class, 'truck_number', 'truck_number');
    }

     public function postman()
    {
        return $this->hasOne(postman::class, 'truck_number', 'truck_number');
    }

}

注意：顺便说一句，我也不能添加$table->foreign('postman_name')->references('postman_name')->on('postmen')->onDelete('cascade');到卡车迁移文件。我这样做时会引发此错误：

SQLSTATE [HY000]：常规错误：1005无法创建表proj_db。trucks（错误号：150“外键约束格式不正确”）