C# – LINQ Group By and select collection


I have this structure

 - has many Orders
  - has many OrderItems

I want to generate a list of CustomerItems via LINQ given a subset of OrderItems:

List of new { Customer, List<OrderItem> Items }

which is a grouping of all the items a Customer has ordered from the subset of items

How can i use LINQ to back track through the order and group by Customer to generate this object?

so far I'm on something like

 .GroupBy(i => i, i => i.Order.Customer, (i, customer) => new {customer, i})

But thats obviously not a List. I'm guessing I need a SelectMany in there somewhere, but could do with some pointers.

Best Solution

I think you want:

items.GroupBy(item => item.Order.Customer)
     .Select(group => new { Customer = group.Key, Items = group.ToList() })

If you want to continue use the overload of GroupBy you are currently using, you can do:

items.GroupBy(item => item.Order.Customer, 
              (key, group) =>  new { Customer = key, Items = group.ToList() })

...but I personally find that less clear.