Linq GROUP BY or manually grouping in a loop? - vb.net

Question

Yesterday I asked this question on stackoverflow. Today I realize that if I do a GROUP BY I also need to create a new type of object.
Let's say I have some data that looks like this:
var1 var2 var3 qty
1 a 1a 50
1 a 1a 25
2 b 2b 10
2 b 2b 15
2 b 2b 10
3 a 3a 25
Here is my working LinQ query
From j In MyTable
Where j.var1 = "xxx"
Group j By Key = New With {Key .var1 = j.var1, Key .var2= j.var2, Key .var3 = j.var3} Into Group
Select New With {.var1 = Key.var1, .var2 = Key.var2, .var3 = Key.var3, .qty = Group.Sum(Function(x) x.qty)}
Actually I use Entity Framework and the code look more like this
Dim foo = (From j In dbContext.MyTable
Where j.var1 = anotherVariable
Group j By Key = New With {Key .var1 = j.var1, Key .var2= j.var2, Key .var3 = j.var3} Into Group
Select New With {.var1 = Key.var1, .var2 = Key.var2, .var3 = Key.var3, .quantity = Group.Sum(Function(x) x.Qty)}).ToArray()
foo is a new type that doesn't exist in my generated Entities. But I have an entity generated by my entity framework that can contains these. It's MyTable itself. I use a GROUP BY only to sum a column of MyTable. I query a MyTable entities and I can put the result in a MyTable entity too.
My question are
1) Can I write something like this
Dim foo = (From j In dbContext.MyTable
Where j.var1 = anotherVariable
Group j By Key = New With {Key .var1 = j.var1, Key .var2= j.var2, Key .var3 = j.var3} Into Group
Select New MyTable With {.var1 = Key.var1, .var2 = Key.var2, .var3 = Key.var3, .qty = Group.Sum(Function(x) x.qty)}).ToArray()
In this case do I need to explicitely write all the mappings ?
2) Should I change my mind. Do a simpler query without GROUP BY and try to group and sum in a VB.NET loop (For Each). Or two queries ? On to get all MyTable with a WHERE clause and another to group ?
Dim foo = dbContext.MyTable.Where(Function(p As MyTable) p.var1 = anotherVariable).ToArray()
For Each bar In foo
'Code to group and sum or another query
Next

Answer 1

You won't be able to instantiate MyTable in an LINQ to Entities query but you can simply enumerate the results of the projection with ToArray and then construct the entities with another Select call.