モデル

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Support\Facades\Auth;

class Cart extends Model
{
    use HasFactory;

    protected $fillable = [
        'stock_id',
        'user_id'
    ];

    public function showCart()
    {
        $user_id = Auth::id();

        $data['my_carts'] = $Cart->where('user_id',$user_id)->get();
        
        return $data;
    }

    public function stock()
    {
        return $this->belongsTo('\App\Models\Stock');
    }

    public function addCart($stock_id)
    {
        $user_id = Auth::id();
        $cart_add_info = Cart::firstOrCreate(['stock_id' => $stock_id,'user_id' => $user_id]);

        if($cart_add_info->wasRecentlyCreated){
            $message = 'カートに追加しました';
        }else{
            $message = 'カートに登録済みです';
        }
        return $message;
    }

}