I've implemented coupons in my app. Coupon is applying on subtotal of all products in the cart. I want to apply the coupon on specific tag's products. There is many-to-many relationship between products and tags. The same tag I'll add while adding a new coupon, and coupon should apply on all the products in this specified tag.

Coupon Model:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;
use App\Tag;

class Coupon extends Model
{
    
    public static function findByCode($code) {
        return self::where('code', $code)->first();
    }

    public function discount($total) {
        if($this->type == 'fixed') {
            return $this->value;
        } else if($this->type == 'percent_off') {
            return ($this->percent_off / 100) * $total;
        } else {
            return 0;
        }
    }
}

Coupons Controller:

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Coupon;
use Cart;

class CouponsController extends Controller
{
    
    public function store(Request $request)
    {
        $this->validate($request, [
            'coupon_code' => 'required'
        ]);
        $coupon = Coupon::where('code', $request->coupon_code)->first();
        $carts = collect([Cart::instance('product')->subtotal(), Cart::instance('specials')->subtotal()])->sum();

        if(!$coupon)
        return redirect()->back()->with('error', 'Invalid Coupon Code');
        session()->put('coupon', [
            'code' => $coupon->code,
            'discount' => $coupon->discount($carts)
        ]);
        
        return redirect()->back()->with('primary' ,'Coupon applied successfully!');
    }

    public function destroy()
    {
        session()->forget('coupon');
        return redirect()->back()->with('warning', 'Coupon has been removed');
    }
}

I will select the tag from admin panel when I'll add new coupon and it should apply on that tag's products.