Orders.cs

static void Main(string[] args)
{
	Mapper.Initialize(cfg =>
	{
		cfg.AllowNullDestinationValues = true;
		cfg.AllowNullCollections = true;
		cfg.SourceMemberNamingConvention = new LowerUnderscoreNamingConvention();
		cfg.DestinationMemberNamingConvention = new PascalCaseNamingConvention();
		cfg.RecognizeDestinationPrefixes("DTO_");

		cfg.CreateMap<Order, OrderDTO>().ForMember(d=>d.OrderLines,o=>o.ExplicitExpansion());
		cfg.CreateMap<OrderLine, OrderLineDTO>();
	});

	using(var context = new TestContext())
	{
		context.Orders.ProjectTo<OrderDTO>(o=>o.OrderLines).Dump();
	}
}

public class DatabaseInitializer : DropCreateDatabaseAlways<TestContext>
{
  protected override void Seed(TestContext context)
  {
  	context.Orders.AddRange(new[]{
		new Order { OrderID = Guid.NewGuid(), OrderLines = { new OrderLine { OrderLineID = Guid.NewGuid() }, new OrderLine { OrderLineID = Guid.NewGuid() }}},
		new Order { OrderID = Guid.NewGuid(), OrderLines = { new OrderLine { OrderLineID = Guid.NewGuid() }, new OrderLine { OrderLineID = Guid.NewGuid() }}},
		new Order { OrderID = Guid.NewGuid(), OrderLines = { new OrderLine { OrderLineID = Guid.NewGuid() }, new OrderLine { OrderLineID = Guid.NewGuid() }}},
	});
  }
}

public class TestContext : DbContext
{
  public TestContext()
  {
      //Database.Log = s=>Debug.WriteLine(s);
      Database.SetInitializer<TestContext>(new DatabaseInitializer());
  }
  public DbSet<Order> Orders { get; set; }
}

public partial class Order
{
   public Order()
   {
       this.OrderLines = new HashSet<OrderLine>();
   }

   public System.Guid OrderID { get; set; }
   public string OrderPlacedBy { get; set; }
   public Nullable<System.DateTime> PlacedTime { get; set; }

   public virtual ICollection<OrderLine> OrderLines { get; set; }
}

public partial class OrderLine
{
   public System.Guid OrderLineID { get; set; }
   public Nullable<System.Guid> OrderID { get; set; }
   public Nullable<double> Amount { get; set; }

   public virtual Order Order { get; set; }
}

public partial class OrderDTO
{
   public OrderDTO()
   {
       this.OrderLines = new HashSet<OrderLineDTO>();
   }

   public System.Guid OrderID { get; set; }
   public string DTO_OrderPlacedBy { get; set; }
   public Nullable<System.DateTime> DTO_PlacedTime { get; set; }

  public virtual ICollection<OrderLineDTO> OrderLines { get; set; }
}

public partial class OrderLineDTO
{
   public System.Guid OrderLineID { get; set; }
   public Nullable<System.Guid> OrderID { get; set; }
   public Nullable<double> DTO_Amount { get; set; }

   public virtual OrderDTO Order { get; set; }
}