Should discount on invoice items and entire invoices be negative line items or separate
properties of an invoice?
In a similar question, Should I incorporate list of fees/discounts into an order class or have them be itemlines, the asker focuses more on orders than invoices (which is a slightly different business entity). Discount is proposed to be separate from order items since it is not equivalent to a fee or product and may have different reporting requirements. Hence, discount should not simply be a negative line item.
Previously I have successfully used negative line items to clearly indicate and calculate discount, but this feels inflexible and inaccurate from a business perspective. Now I am opting to add discount to each line item, along with an invoice-wide discount.
Is this the right way to do it?
Should each item have its own discount amount and percentage?
Domain Model Code Sample
This is what my domain model, which maps to an SQL repository, looks like:
public class Invoice
{
public int ID { get; set; }
public Guid JobID { get; set; }
public string InvoiceNumber { get; set; }
public Guid UserId { get; set; } // user who created it
public DateTime Date { get; set; }
public decimal DiscountPercent { get; set; } // all lines discount %?
public decimal DiscountAmount { get; set; } // all lines discount $?
public LazyList<InvoiceLine> InvoiceLines { get; set; }
public LazyList<Payment> Payments { get; set; } // for payments received
public boolean IsVoided { get; set; } // Invoices are immutable.
// To change: void -> new invoice.
public decimal Total
{
get {
return (1.0M - DiscountPercent)
* InvoiceLines.Sum(i => i.LineTotal)
- DiscountAmount;
}
}
}
public class InvoiceLine
{
public int ID { get; set; }
public int InvoiceID { get; set; }
public string Title { get; set; }
public decimal Quantity { get; set; }
public decimal LineItemPrice { get; set; }
public decimal DiscountPercent { get; set; } // line discount %?
public decimal DiscountAmount { get; set; } // line discount amount?
public decimal LineTotal {
get {
return (1.0M - DiscountPercent)
* (this.Quantity * (this.LineItemPrice))
- DiscountAmount;
}
}
}
