Entity Framework – Eager loading of subclass related objects

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This is a solution which requires only a single roundtrip:

var orders = Context.Orders
    .Select(o => new
    {
        Order = o,
        SubOrderBases = o.SubOrderBases.Where(s => !(s is MyOrder)),
        MyOrdersWithCustomers = o.SubOrderBases.OfType<MyOrder>()
            .Select(m => new
            {
                MyOrder = m,
                Customers = m.Customers
            })
    })
    .ToList()  // <- query is executed here, the rest happens in memory
    .Select(a => 
    {
        a.Order.SubOrderBases = new List<SubOrderBase>(
            a.SubOrderBases.Concat(
            a.MyOrdersWithCustomers.Select(m => 
                {
                    m.MyOrder.Customers = m.Customers;
                    return m.MyOrder;
                })));
        return a.Order;
    })
    .ToList();

It is basically a projection into an anonymous type collection. Afterwards the query result is transformed into entities and navigation properties in memory. (It also works with disabled tracking.)

If you don’t need entities you can omit the whole part after the first ToList() and work directly with the result in the anonymous objects.

If you must modify this object graph and need change tracking, I am not sure if this approach is safe because the navigation properties are not completely set when the data are loaded – for example MyOrder.Customers is null after the projection and then setting relationship properties in memory could be detected as a modification which it isn’t and cause trouble when you call SaveChanges.

Projections are made for readonly scenarios, not for modifications. If you need change tracking the probably safer way is to load full entities in multiple roundtrips as there is no way to use Include in a single roundtrip to load the whole object graph in your situation.

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