osb/app/Http/Controllers/UserController.php

<?php

namespace App\Http\Controllers;

use Carbon\Carbon;
use Illuminate\Http\Request;
use Illuminate\Support\Facades\Session;
use Illuminate\Validation\Rule;

use App\Models\{Supplier,User};

class UserController extends Controller
{
	/**
	 * Add a supplier to a user's profile
	 *
	 * @param Request $request
	 * @param User $o
	 * @return \Illuminate\Http\RedirectResponse
	 */
    public function supplier_addedit(Request $request,User $o)
	{
		Session::put('supplier_update',true);

		$validated = $request->validate([
			'id'=> ['required','string',Rule::unique('supplier_user')->where(fn ($query) => $query->where('supplier_id',$request->supplier_id)->where('user_id','<>',$o->id))],
			'supplier_id'=>'required|exists:suppliers,id',
		]);

		$o->suppliers()->attach([
			$validated['supplier_id'] => [
				'id'=>$validated['id'],
				'site_id'=>$o->site_id,
				'created_at'=>Carbon::now(),
			]
		]);

		return redirect()
			->back()
			->with('success','Supplier Added');
	}

	/**
	 * Remove a supplier from a user's profile
	 *
	 * @param User $o
	 * @param Supplier $so
	 * @return \Illuminate\Http\RedirectResponse
	 */
	public function supplier_delete(User $o,Supplier $so)
	{
		Session::put('supplier_update',true);

		$o->suppliers()->detach([$so->id]);

		return redirect()
			->back()
			->with('success','Supplier Deleted');
	}
}