Implementing Business Logic - ASP.NET Dynamic Data - page 288

Product's UnitsInStock , it doesn't work because an Order_Detail object might not change
when the order is submitted. Instead, it is better to validate all Order_Detail objects in a
particular order at once when the order status changes to Submitted. Not only does this
approach allow you to validate the unchanged Order_Detail objects, it is usually more
efficient from the database access standpoint as well. Here is an extract from the
ValidateSubmittedOrder method in Listing 9.1 that performs this validation:
Product product = this.Order_Details
.Where(item => item.Product.UnitsInStock < item.Quantity)
.Select(item => item.Product).FirstOrDefault();
if (product != null)
yield return new ValidationResult(product.ProductName + “ is out of
stock”);
To check for out-of-stock products in an order, the ValidateSubmittedOrder method iter-
ates over all Order_Detail objects of the order, comparing their Quantity with the
number of UnitsInStock of the product. If any order item does not have enough product
units available, a ValidationResult is returned to indicate an error.
This code relies on the lazy loading , which is enabled by default in the NorthwindEntities
class, to access Product entity objects. As you recall from our previous discussion, lazy
loading makes writing code easier at the cost of performance. Here is how we could
rewrite the Order validation logic to determine if it has any out-of-stock items in a single
database request (remember that context is the ValidationContext object passed to the
ValidateSubmittedOrder method as a parameter):
var entities = (NorthwindEntities)context.Items[typeof(ObjectContext)];
string productName = entities.Order_Details
.Where(item => item.OrderID == this.OrderID
&& item.Product.UnitsInStock < item.Quantity)
.Select(item => item.Product.ProductName).FirstOrDefault();
if (productName != null)
yield return new ValidationResult(productName + “ is out of stock”);
Although the new LINQ query looks similar to the previous version, the key difference is
that this new version uses the Order_Details property of the NorthwindEntities class,
which returns an IQueryable<Order_Detail> instead of the Order_Details property of the
Order class, which returns an IEnumerable<Order_Detail> . As a result, instead of using
LINQ to Objects and iterating over the lazy-loaded Order_Detail collection in memory of
the application, you now use LINQ to Entities to send a single request the database server,
query the Order_Details and Products tables, and return the name of the first out-of-stock
product.
The ValidateSubmittedOrder method retrieves an instance of the NorthwindEntities
object context from the ValidationContext object. As discussed in Chapter 8, the
ValidationContext allows the UnleashedObjectContext to pass the ObjectStateEntry object
that stores the entity itself to the validation methods. To make the NorthwindEntities object
context available as well, the Validate method of the UnleashedObjectContext class was
further extended as shown in Listing 9.2.