ldc-jeusselin/app/Models/BusinessSector.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class BusinessSector extends Model
{
    use HasFactory;

    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
        'name',
        'order',
        'parent_id',
    ];

    public function employees()
    {
        return $this->belongsToMany(Employees::class, 'sectors_employees', 'business_sector_id', 'employee_id')->orderBy('order', 'ASC');
    }

    public function parent()
    {
        return $this->hasOne(self::class, 'id', 'parent_id');
    }

    public function children()
    {
        return $this->hasMany(self::class, 'parent_id', 'id');
    }
}