MVC Entity Framework Partial Class access DB for property value - asp.net-mvc

Question

I am using Entity Framework mapped to my database. I have a Basket model which can have many BasketItem models, and I have Promotions and Coupons models.
This is for eCommerce checkout functionality and I just don't understand how this will work, here goes:
Because my BasketItems have a foreign key relationship to the Basket if I want to sum up the Subtotal for my basket items in a partial class, I can do this:
public decimal Subtotal {
get {
return this.BasketItems.Sum(pb => pb.Subtotal);
}
}
This is helpful because I can use this inside a view, there's no mucking around with passing a DB context through and it's DRY, etc. etc.
Now I want to apply promotions or coupons to my Subtotal ideally I want it to look like this:
public decimal DiscountedSubtotal {
get {
decimal promotions_discount = 0;
decimal coupons_discount = 0;
return Subtotal - promotions_discount - coupons_discount;
}
}
But there is no access to Promotions or Coupons without either creating some crazy and unnecessary relationships in my database or some light hacking to get this functionality to work. I don't know what I should do to overcome this problem.
Solution 1:
public decimal DiscountedSubtotal(DatabaseEntities db) {
decimal promotions_discount = from p in db.Promotions
select p.Amount;
decimal coupons_discount = from c in db.Coupons
select c.Amount;
return Subtotal - promotions_discount - coupons_discount;
}
I don't want to use this in my View pages, plus I have to send through my context every time I want to use it.
Solution 2: (untested)
public List<Promotion> Promotions { get; set; }
public List<Coupon> Coupons { get; set; }
public Basket()
: base() {
DatabaseEntities db = new DatabaseEntities();
Promotions = db.Promotions.ToList();
Coupons = db.Coupons.ToList();
}
A bit of light hacking could provide me with references to promotions and coupons but i've had problems with creating new contexts before and I don't know if there is a better way to get me to the DiscountedSubtotal property I would ideally like.
So to sum up my question, I would like to know the best way to get a DiscountedSubtotal property.
Many thanks and apologies for such a long read :)

Answer 1

I think the problem here is that you're not really using a coherent architecture.
In most cases, you should have a business layer to handle this kind of logic. Then that business layer would have functions like CalculateDiscountForProduct() or CalculateNetPrice() that would go out to the database and retrieve the data you need to complete the business rule.
The business class would talk to a data layer that returns data objects. Your view only needs it's view model, which you populate from the business objects returned by your businesss layer.
A typical method might be:
public ActionResult Cart() {
var model = _cartService.GetCurrentCart(userid);
return View(model);
}
So when you apply a discount or coupon, you would call a method like _cartService.ApplyDiscount(model.DiscountCode); then return the new model back to the view.
You might do well to study the Mvc Music Store sample project, as it includes cart functionality and promo codes.
http://www.asp.net/mvc/tutorials/mvc-music-store/mvc-music-store-part-1