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 give

Answer 1

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.