Question

I am using Entity Framework 4.1 code first+MVC3 and the inheritence stratagy that I use is TPC

I have the following classes

Public Class ObjectBase
    <Key()>
    Public Property Id As Integer

    Public Property Description As String
End Class

Public Class Computer
    Inherits ObjectBase

    Public Property Computername As String
End Class

Public Class Book
    Inherits ObjectBase

    Public Property BookName As String
End Class

Public Class User
    <Key()>
    Public Property Id As Integer

    Public Property Name As String
End Class

Public Class BorrowObject
    <Key()>
    Public Property Id As Integer

    Public Property User As User

    Public Property BorrowedObject as ObjectBase
End Class

Public Class BorrowComputerVM
    <Key()>
    Public Property Id As Integer

    Public Property User As User

    Public Property Computer as Computer
End Class

My questions are:

1. How do I do a query (using LINQ,
   Entity SQL or other commonly used
   way) to get all BorrowObjects where
   BorrowedObject is of type Computer?
2. How do I map the result of the query
   to the ViewModel called
   “BorrowComputerVM” (used for
   creating views only used for
   borrowing a Computer).

Question 1 (and question 2) should be very simple, but I have allready spent hours on Google to find an answer with no result at all. The only thing I have found is that you can get all computers in ObjectBase by writing context.ObjectBase.OfType(Of Computer), and that does not help since you cannot write context.BorrowObjects.ObjectBase.OfType(Of Computer)

Please provide code samples in VB.NET (if you can), but more importantly: Please ensure that the codesamples you supply work without hours of modification!

Answer 1

I write it in C#:

How do I do a query (using LINQ, Entity SQL or other commonly used way)
to get all BorrowObjects where
BorrowedObject is of type
Computer?

var list = context.BorrowObjects.Include(b => b.BorrowedObject)
                  .Where(b => b.BorrowedObject is Computer)
                  .ToList();

You can omit the Include if you don’t want to eager load the Computer.

How do I map the result of the query to the ViewModel called
“BorrowComputerVM” (used for creating
views only used for borrowing a
Computer).

var list = context.BorrowObjects
                  .Where(b => b.BorrowedObject is Computer)
                  .Select(b => new BorrowComputerVM
                          {
                              Id = b.Id,
                              User = b.User,
                              Computer = b.BorrowedObject as Computer
                          })
                  .ToList();

Includes are not necessary if you project into a new type: User and Computer will be loaded in the BorrowComputerVM also without explicit Include.

The is and as operators work indeed in LINQ to Entities.