public function parent()

    {

        return $this->belongsTo('App\CourseModule','parent_id')->where('parent_id',0);

    }

​

    public function children()

    {

        return $this->hasMany('App\CourseModule','parent_id');

    }

​

namespace App\Models;

​

use Illuminate\Database\Eloquent\Factories\HasFactory;

use Illuminate\Database\Eloquent\Model;

​

class Test extends Model

{

    use HasFactory;

​

    protected $parentColumn = 'parent_id';

​

    public function parent()

    {

        return $this->belongsTo(Test::class,$this->parentColumn);

    }

​

    public function children()

    {

        return $this->hasMany(Test::class, $this->parentColumn);

    }

​

    public function allChildren()

    {

        return $this->children()->with('allChildren');

    }

​

}

​