Errata

Regarding while lazy loading is True, and the paragraph near the middle saying:

'In this example, the order detail objects were not retrieved until the ToList method was executed.'

I did a similar test (lazy loading = true) using:

dg1.ItemsSource = (from p in db.People
select p).ToList();

That cause an ObjectDisposedException. According to the MSDN link here

http://msdn.microsoft.com/en-us/library/system.data.objects.objectcontext.connection.aspx

the lazy loading occurs when a navigation method is used. So I changed my test to:

dg1.ItemsSource = (from p in db.People
select p.StudentGrades).ToList();

And now it works! So the part in middle of page 383 that says

'In this example, the order detail objects were not retrieved until the ToList method was executed.'

should mention how the navigation method (Order_Details in the book example) is crucial. This may seem minor, but it is nice to know how to avoid ObjectDisposedExceptions while lazy loading is true.

This regards the example code which uses a custom property 'od.DetailTotal'.

When I tried this I got the error 'The specified member DetailTotal' is not supported in LINQ to Entities. Only initializers, entity members, and entity navigation can ... etc.

So I did some research and found a forum link called "EF4 CTP4 Code First - The specified type member 'Total' is not supported in LINQ to Entities" which explained how:

The issue is that the EF LINQ Provider will take the query you write and convert it into to SQL that then runs against the database. Because the logic for calculating your custom property is only defined in code, EF has no way to know how to translate this into SQL. So need to write the query to pull the orders and details into memory so that you can use your custom property then.

Below is what worked for me. Note, I could not use the navigation method od.Order.CustomerID (like on p. 390) as 'Order' was always null for some reason, so I used a different method to get the orders.

// Execute SQL DB query now
var OrderDetails = (from o in db.Orders
where o.CustomerID == mSelectedCustomerID
join od in db.Order_Details
on o.OrderID equals od.OrderID
select od).ToList();

// Now we can use our custom property via in-memory collection:
dg1.ItemsSource = from od in OrderDetails
select new
{
od.OrderID,
od.ProductID,
od.Quantity,
od.Discount,
TotalPrice = od.DetailTotal.ToString("c") // our custom property
};