Automapper projection results in empty collection for nested Dto - automapper

Question

I have a .Net Core 2 webapi in which I am using automapper to map to Dtos. Everything works fine, except I am seeing an unexpected behaviour when I map an object to a Dto, and where the Dto also contains mappings for a collection. E.g
CreateMap<Order, OrderDto>();
CreateMap<Product, ProductDto>();
Where classes are like this
public partial class Order
{
public int Id { get; set; }
public string Name { get; set; }
public virtual ICollection<Product> Products{ get; set; }
public int ProductCount {return Products.Count;}
}
public partial class Product
{
public int Id { get; set; }
public int OrderId { get; set; }
public string Name { get; set; }
public string Description { get; set; }
}
The following works as expected;
The class is mapped, and the ProjectCount is correct in the Dto
public partial class OrderDto
{
public int Id { get; set; }
public virtual ICollection<Product> Products{ get; set; }
public int ProductCount{ get; set; }
}
_context.Orders.Include<>(Products).ProjectTo<>(OrderDto)
But doing the following, the productcount is always zero.
E.g. if I do this;
public partial class OrderDto
{
public int Id { get; set; }
public virtual ICollection<ProductDto> Products{ get; set; }
public int ProductCount{ get; set; }
}
public partial class ProductDto
{
public int Id { get; set; }
public int OrderId { get; set; }
public string Name { get; set; }
}
_context.Orders.Include<>(Products).ProjectTo<>(OrderDto)
Why does this happen, and how can I ensure that it doesnt? This is a real world example where I need a property which references the collection - and I need it in both the base and the Dto. I can do the following which works fine, but it doesnt appear that this should be how it works...
public partial class OrderDto
{
public int Id { get; set; }
public virtual ICollection<ProductDto> Products{ get; set; }
public int ProductCount {return Products.Count;}
}
public partial class ProductDto
{
public int Id { get; set; }
public string Name { get; set; }
}
_context.Orders.Include<>(Products).ProjectTo<>(OrderDto)

Answer 1

I profiled the SQL and found that Automapper changes the way the query is formed. Without the nested projection, two queries are made;
[Queries are more complex than this and use joins, but you get the idea]
Select Id from orders
Select Id,Name from products where productid in [select id from orders ]
With the nested projection, are executed for each nested Dto
Select Id from orders
Select Id,Name from products where id=1
Select Id,Name from products where id=2