AutoMapper mapping collection to nested objects collection

Question

I am trying to map an object with a collection property to another object with a nested class containing mapped versions of the source collections, but I can't seem to figure out the appropriate mapping required.

Below are my class and attempted mappings.

public class TypeA
{
    public int Id;
    public string Description;
}

public class TypeB
{
    public int Id;
    public int Code;
}

public class Source
{
    public ICollection<TypeA> A;
    public ICollection<TypeB> B;
}

public class Destination
{
    public DestinationInner Inner;
}

public class DestinationInner
{
    public ICollection<int> A;
    public ICollection<int> B;
}

I have the following mappings defined, which works fine when the destination collection is at the parent level and not in the DestinationInner class.

Mapper.CreateMap<TypeA, int>()
          .ConvertUsing(a => a.Id);

Mapper.CreateMap<TypeB, int>()
          .ConvertUsing(b => b.Id);

I've tried adding a map such as, but I can't find a way of forcing a mapping here on the collections

Mapper.CreateMap<Source, Destination>()
          .ForMember(d => d.Inner,
                     opt.MapFrom(s => new DestinationInner()
                                      {
                                          // not sure if I can assign anything here
                                          A = 
                                          B = 
                                      }));

Is MapFrom the appropriate method to use here - or am I approaching this from the wrong direction?

The alternative is to not use AutoMapper for the collections and just use Linq, but I'm thinking there has to be a way to fully use AutoMapper:

Mapper.CreateMap<Source, Destination>()
          .ForMember(d => d.Inner,
                     opt.MapFrom(s => new DestinationInner()
                                      {
                                          A = s.A.Select(a => a.Id).ToList(),
                                          B = s.B.Select(b => b.Id).ToList(),
                                      }));

Answer 1

Lucian provided the answer in the comments:

ForPath(d => d.Inner.A, o => o.MapFrom(s => s.A))