Implementing Business Logic - ASP.NET Dynamic Data - page 276

This section discusses these approaches and some of the trade-offs you must take
into consideration when choosing how to implement entity interaction rules in your
application.
Implementing Entity Interaction Rules in BeforeSave Methods
You can use the BeforeSave “trigger” methods to implement not only property interaction
rules, but entity interaction rules as well. For instance, you could change the BeforeSave
method of the Order entity class to detect if the OrderStatus property changed from Paid
to Fulfilled and update the UnitsInStock property of all Product entities purchased in
this Order. The code example that follows illustrates how the implementation could look:
partial class Order : ISavableObject
{
public void BeforeSave(ObjectStateEntry stateEntry)
{
byte? oldStatus = (byte)DataModel.OrderStatus.Draft;
if (stateEntry.State == EntityState.Modified)
oldStatus = stateEntry.OriginalValues.Get(() => this.OrderStatus);
if (this.OrderStatus == (byte)DataModel.OrderStatus.Submitted &&
oldStatus == (byte)DataModel.OrderStatus.Draft)
{
this.ShippedDate = DateTime.Now;
foreach (Order_Detail orderItem in this.Order_Details)
orderItem.Product.UnitsInStock -= orderItem.Quantity;
}
}
}
Although this approach is viable for trivial scenarios such as in this example, it quickly
becomes overwhelmingly difficult to maintain as the number of rules and their complex-
ity increases. One of the reasons this happens is because the code is guessing what the
presentation layer is trying to accomplish. When only the OrderStatus property is chang-
ing from Paid to Fulfilled , it is easy to see what is going on, but what if another prop-
erty of the Order entity changes as well? For example, you could have another business
rule tied to the ShipVia property of the Order entity to calculate the shipping fee (the
Freight property) depending on the shipping option selected by the customer. If the
OrderStatus changes to Fulfilled at the same time with the ShipVia property, does it
mean an employee has chosen a different shipper to expedite the customer's order at no
additional cost or that the customer stopped by to pick up the order from our physical
store and you need to refund the shipping fee? It is impossible to answer this question in
the business layer of the application because it depends on how the presentation layer is
implemented.