Php groupBy方法可以'；t插入或保存多个值

我在想为什么我不能在我做的语句中插入或保存我分组的表。我对它们进行了适当的分组，但当我试图将其插入数据库时，它只得到列中的第一个值，而不是所有值。我在部门和用户

倒模

正如您在“工程部”中看到的，我有一个用户id，它是3和4。当我试图将其插入数据库时，第一个值只是插入的值

数据库值

控制器

public function getDocuments()
 {
   $departmentRecipient = DB::table('users')->select('departments.department_name', 'users.id', DB::raw('group_concat(users.id)'))
    ->join('departments', 'departments.id', '=', 'users.department_id')
    ->groupBy('departments.department_name')
    ->get();
 }

public function postDocuments(Request $request)
{


    $document = new Document();
                                //Request in the form
    $document->title = $request->title;
    $document->content = $request->content;
    $document->category_id = $request->category;

    $document->save();

    $user = Auth::user();

    foreach($request->recipient as $recipientId)
    {
        $document->sentToUsers()->sync([ $recipientId => ['sender_id' => $user->id]],false );
    }

}

看法

收件人：
@foreach（$departmentRecipient作为$list）
{{$list->department_name}
@endforeach

<div class = "form-group">

    <label for = "recipient" class = "control-label">Recipient:</label>

    <select name = "recipient[]" multiple class = "form-control select2-multi" id = "myUserList">

        @foreach ($departmentRecipient as $list)
            <option value = "{{ $list->id }}">{{ $list->department_name }}</option>
        @endforeach

    </select>

</div>