LINQ Group By and select collection

Question

I have this structure

Customer  - 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

items  .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.

Answer 1

I think you want:

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

...but I personally find that less clear.