Возможно, вы можете использовать resolver в ResolveUsing () Automapper.Например:
Mapper.CreateMap()
.ForMember(x=>x.DocumentDetails, opt=>opt.ResolveUsing<DocumentDetailsResolver>()
.FromMember(src=>src.Document.DocumentId);
Затем в вашем классе DocumentDetailsResolver:
public class DocumentDetailsResolver: ValueResolver<int, List<DocumentDetails>{
protected override List<DocumentDetails> ResolveCore(int source)
{
// Put your logic to get the list of document details, source is the ID of the document
// Return the list
}
}