[c#] LINQ Group By and select collection

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.

you can achive it with group join

var result = (from c in Customers
          join oi in OrderItems on c.Id equals oi.Order.Customer.Id into g
          Select new { customer = c, orderItems = g});

c is Customer and g is the customers order items.

you may also like this

var Grp = Model.GroupBy(item => item.Order.Customer)
      .Select(group => new 
        { 
             Customer = Model.First().Customer, 
             CustomerId= group.Key,  
             Orders= group.ToList() 
       })
      .ToList();