How to use LINQ to retrieve child collection

Question

Starting here:

public class Customer
{
  public int CustomerID { get; set; }
  public string CustomerName { get; set; }  
  public IList<Order> Orders { get; set; }
}

public class Order
{
  public int OrderID { get; set; }
  public int CustomerID { get; set; }
}

What would be the linq query that you write to retrieve all Orders from all Customers?

Something to the effect of:

IList<Order> orders = new List<Order>();
foreach (Customer c in Customers)
{
    orders.Union(c.Orders);
}

I tried the following but it throws an "nhibernate collection was not an association" exception:

var orders = from c in Customers
             select c.Orders;

I know I'm missing something, but I can't find a way to achieve this in linq.

Answer 1

var orders = from c in db.Customers
    from o in c.Orders
    select o;

Or

var orders = db.Customers.SelectMany(c => c.Orders);

Will it work in linq for nhibernate? I don't know.

Answer 2

Make sure your foreign key constraint is setup properly from [Order].[CustomerID] to [Customer].[ID] in the database.