allo/app/Model/StaffDetail.php

<?php

namespace App\Model;

use Jenssegers\Mongodb\Eloquent\SoftDeletes;
use Jenssegers\Mongodb\Eloquent\Model as Eloquent;

class StaffDetail extends Eloquent {
    //
	use SoftDeletes;
    
	protected $connection = 'mongodb';
    protected $collection = 'staff_detail';
	
	// protected $guarded = ['_id'];
	protected $fillable = ['_id'];

	public function staff() {
        return $this->belongsTo('App\Staff','_id','_id')->withTrashed();
    }
}