AutoMapper smoothes nested collections

Question

I am trying to figure out how to flatten a collection of Merchants, each of which contains a collection of Orders for a flat list of OrderViewModels.

Here is my DTO:

public class Merchant
{
    public string MerchantName { get; set; }
    public List<Order> Orders { get; set; }
}

public class Order
{
    public string OrderId { get; set; }
}

And here is the view model:

public class OrderViewModel
{
    public string MerchantName { get; set; }
    public string OrderId { get; set; }
}

My goal is to flatten the <Merchant> List to the <OrderViewModel> list , while the following testing structure should lead to 6 view models:

var myMerchants = new List<Merchant>
{
    new Merchant
    {
        MerchantName = "Merchant X",
        Orders = new List<Order>
        {
             new Order { OrderId = "Order 1"},
             new Order { OrderId = "Order 2"},
             new Order { OrderId = "Order 3"}
        }
    },
    new Merchant
    {
        MerchantName = "Merchant Y",
        Orders = new List<Order>
        {
            new Order { OrderId = "Order 4"},
            new Order { OrderId = "Order 5"},
            new Order { OrderId = "Order 6"}
        }
    }
 };

 var models = Mapper.Map<List<OrderViewModel>>(myMerchants);

Answer 1

1:1 (.. 2 root Merchants 6 OrderViewModels), custom TypeConverter , .SelectMany :

public class MyTypeConverter : ITypeConverter<IEnumerable<Merchant>, List<OrderViewModel>>
{
    public List<OrderViewModel> Convert(ResolutionContext context)
    {
        if (context == null || context.IsSourceValueNull)
            return null;

        var source = context.SourceValue as IEnumerable<Merchant>;

        return source
            .SelectMany(s => s.Orders
              .Select(o => new OrderViewModel
              {
                  MerchantName = s.MerchantName,
                  OrderId = o.OrderId
              }))
              .ToList();
    }
}

:

Mapper.CreateMap<IEnumerable<Merchant>, List<OrderViewModel>>()
    .ConvertUsing<MyTypeConverter>();

:

var models = Mapper.Map<List<OrderViewModel>>(myMerchants);

Answer 2

, automapper.

var models = myMerchants.SelectMany(s => s.Orders.Select(o => new OrderViewModel { MerchantName = s.MerchantName, OrderId = o.OrderId })).ToList();