subCategories function is defiened but all what I get is Call to undefined method Illuminate\Database\Eloquent\Builder::subCategories()"

I have category table which has parent_category_id column since each category can have more than sub category so I in my Category model:

namespace App;

use Illuminate\Database\Eloquent\Model;

class Category extends Model
{
    //
    public function subCategories(){
        $this->hasMany('App\Category', 'parent_category_id');
    }
    public function products(){
        $this->hasMany('App\Product');
    }
    public function parentCategory(){
        $this->belongsTo('App\Category', 'parent_category_id');
    }

but when I try Category::where('parent_category_id', null)->where('is_hidden', 0)->subCategories()->get()

all I get Call to undefined method Illuminate\Database\Eloquent\Builder::subCategories()"

how to solve this?

via Chebli Mohamed