Entity Framework how to derive from a non existent class

Question

Please bear with me, I am not a developer, so I apologise if I don't use the correct terminology, but I am building a very simple stock management app using code first EF. I have one class which is my Products (simplified for brevity):

public class Product()
{
    this.Shops = new HashSet<Shop>
}
public int Id { get; set; }
public string ProductDescription { get; set; }
public virtual ICollection<Shop> Shops { get; set; }

And my Shop class:

public Shop() 
{
    this.Products = new HashSet<Product>();
}
public int Id { get; set; }
public string ShopName { get; set; }
public virtual ICollection<Product> Products { get; set; }

Not all products will be available at all shops, and the above works perfectly. EF has created the mapping table ProductShop and I can make whatever products I like available at any of the shops.

My problem is how to deal with the quantity of each item that is available at each shop. Initially I thought I could just derive a new class from the ProductShop entity with a quantity field like this:

public class ShopStockQuantities : ProductShop
{
    public int Qty { get; set; }
}

but I don't have a physical class to derive from. If I add that class, will EF get in a knot as that table already exists? I am sure this issue has been encountered before, so to save me hours of experimenting (as I am not very good at this anyway) I thought I would seek advice from this Borg of experts. Any advice gratefully received.

Answer 1

Based on the information you've provided, this is one possible class structure which basically reflects what Wiktor and HH are saying.

public class Product
{
    [Key] public int ProductId { get; set; } 
    public string ProductDescription { get; set; } = "Not Set";
}

public class Shop
{
    [Key] public int ShopId { get; set; }
    public string ShopName { get; set; } = "Not Set";
    public ICollection<Stock> StockItems { get; set; } = new Collection<Stock>();
}

public class Stock
{
    [Key] public int StockId { get; set; }
    public int ShopId { get; set; }  // foreign key link to Shop
    public int ProductId { get; set; }   // foreign key to Product
    public int Quantity { get; set; }
}