Members with custom mapping definitions not populated unless $select is used

[jazzmanro]

Starting from the samples in this repo, let's make OpsTenant.BuildingIds a List<int> and add a mapping:

Source/destination types

    public class OpsTenant : BaseOpsTenant
    {
        [Key]
        public Guid Identity { get; set; }
        public DateTime CreatedDate { get; set; }
        public List<int> BuildingIds { get; set; }
        public ICollection<CoreBuilding> Buildings { get; set; }
    }

Mapping configuration

            CreateMap<TMandator, OpsTenant>()
                .ForMember(d => d.BuildingIds, o => o.MapFrom(s => s.Buildings.Select(b => b.Id).ToList()))
                .ForAllMembers(o => o.ExplicitExpansion());

Version: 4.0.1

Expected behavior

BuildingIds should be populated when calling:
http://localhost:16324/opstenant

Actual behavior

BuildingIds is not populated by default unless I explicitly $select it:
http://localhost:16324/opstenant?$select=Identity,Name,CreatedDate,BuildingIds

I think this is an issue as it is not consistent with the other members behavior. BuildingIds is a simple member with a custom mapping, it is neither a navigation nor complex property, $expand won't work. To be forced to $select in order to access the data is not intuitive and also not practical as it requires to explicitly $select all other properties in order to get the default structure.

[BlaiseD]

That's by design. With explicit expansion only the literal types will expand by default.

If you're interested consider the approach we use for including literal types and include a PR for the non-navigation/simple members.