View file app/Models/Board.php

<?php

namespace App\Models;

class Board extends BaseModel
{

    /**
     * Indicates if the model should be timestamped.
     *
     * @var bool
     */
    public $timestamps = false;

    /**
     * The attributes that aren't mass assignable.
     *
     * @var array
     */
    protected $guarded = [];

    /**
     * Директория загрузки файлов
     *
     * @var string
     */
    public $uploadPath = UPLOADS . '/boards';

    /**
     * Возвращает связь родительской категории
     */
    public function parent()
    {
        return $this->belongsTo(self::class, 'parent_id')->withDefault();
    }

    /**
     * Возвращает связь подкатегорий
     */
    public function children()
    {
        return $this->hasMany(self::class, 'parent_id')->orderBy('sort');
    }
}