Help me understand entity framework 4 caching for lazy loading

Posted by Chris on Stack Overflow See other posts from Stack Overflow or by Chris
Published on 2010-04-27T21:07:38Z Indexed on 2010/04/29 19:57 UTC
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I am getting some unexpected behaviour with entity framework 4.0 and I am hoping someone can help me understand this. I am using the northwind database for the purposes of this question. I am also using the default code generator (not poco or self tracking). I am expecting that anytime I query the context for the framework to only make a round trip if I have not already fetched those objects. I do get this behaviour if I turn off lazy loading. Currently in my application I am breifly turning on lazy loading and then turning it back off so I can get the desired behaviour. That pretty much sucks, so please help. Here is a good code example that can demonstrate my problem.

  Public Sub ManyRoundTrips()
    context.ContextOptions.LazyLoadingEnabled = True
    Dim employees As List(Of Employee) = context.Employees.Execute(System.Data.Objects.MergeOption.AppendOnly).ToList()

    'makes unnessesary round trip to the database, I just loaded the employees'
    MessageBox.Show(context.Employees.Where(Function(x) x.EmployeeID < 10).ToList().Count)
    context.Orders.Execute(System.Data.Objects.MergeOption.AppendOnly)
    For Each emp As Employee In employees
        'makes unnessesary trip to database every time despite orders being pre loaded.'
        Dim i As Integer = emp.Orders.Count
    Next
End Sub

Public Sub OneRoundTrip()
    context.ContextOptions.LazyLoadingEnabled = True
    Dim employees As List(Of Employee) = context.Employees.Include("Orders").Execute(System.Data.Objects.MergeOption.AppendOnly).ToList()

    MessageBox.Show(employees.Where(Function(x) x.EmployeeID < 10).ToList().Count)

    For Each emp As Employee In employees
        Dim i As Integer = emp.Orders.Count
    Next
End Sub

Why is the first block of code making unnessesary round trips?

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