使用此代码中的递归可以解决什么问题？

Question

BadMethodCallException in Builder.php line 2405:
Call to undefined method Illuminate\Database\Query\Builder::customers()

我的服务班级：

namespace App;

use Illuminate\Database\Eloquent\Model;

class Service extends Model
{
         protected $fillable = [
        'title', 'type','link','description','customer_id','user_id'
    ];
        public $timestamps = false;

        public function customers(){
        return $this->belongsTo('App\Customer');

    }
        public function user(){
        return $this->belongsTo('App\User');

    }
}

我的ServiceController类：

class ServiceController extends Controller
{
        public function store(Request $request)
    {
        $validator = Validator::make($request->all(), [
            'title' => 'required|unique:customers|max:255',
            'type' => 'required',
            'link' => 'required',
            'description' => 'required',

        ]);

        $services = new Service();
        $services->title = $request['title'];
        $services->type = $request['type'];
        $services->link = $request['link'];
        $services->description = $request['description'];
        $services->customer_id = Customer::customers()->id;
        $services->save();

        return redirect('/services');
    }