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

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