laravel eloquent ->with not working with one to one - php

Question

now i have this method inside my Product model
public static function product_lines($request)
{
$keyword = Helper::clean_keyword($request['keyword']);
$data = Product::where
('number','like','%'.$keyword.'%')->
orWhere('name','like','%'.$keyword.'%')->
with('get_unit_relations')-> // this is one to many
limit(30)->
get();
return $data;
}
and this is the get_unit_relations
public function get_unit_relations()
{
return $this->hasMany('App\Models\Product_unit','product_id','id');
}
like this its working and its return data without any problem ..
{
"id": 1,
"number": "1",
"name": "test",
"foreign_name": null,
"get_unit_relations": [
{
"id": 3,
"unit_id": 2,
"product_id": 1,
"barcode": "455",
"smallest_unit_relation": 12,
"price": 12,
"cost": null,
"created_by": null,
"deleted_by": null,
"created_at": "2020-05-17T21:46:26.000000Z",
"updated_at": "2020-05-17T21:46:26.000000Z",
"deleted_at": null
}
]
}
but if i change the code like this ..
public static function product_lines($request)
{
$keyword = Helper::clean_keyword($request['keyword']);
$data = Product::where
('number','like','%'.$keyword.'%')->
orWhere('name','like','%'.$keyword.'%')->
with('get_smallest_unit_id')-> // this is one to one
limit(30)->
get();
return $data;
}
and this is the get_smallest_unit_id
public function get_smallest_unit_id()
{
return $this->hasOne('App\Models\Unit','id','smallest_unit_id');
}
im getting null in return like this ..
{
"id": 1,
"number": "1",
"name": "test",
"foreign_name": null,
"get_smallest_unit_id": null // it always return null in any one to one relaiton ..
}
how can i use ->with() method with one to one relation
thanks