EF6 nested collection count (without fetching) with/without automapper - entity-framework-6

Question

According to this documentation page there is a neat way to count nested collection elements without actually fetching them. Great. But the example is for a single item.
Of course, I could iterate and do this in post-processing. But that's not neat.
And here could something like Automapper come into play. It has a ProjectTo extension for IQueryable which can be parameterized (in quite a weird way imho).
Well, I can't get it working. Let's say I have a Client entity that has associated Device entities, and I want to set the DeviceCount property of the target.
By combining the examples from the two sources, something like this could work:
_configuration = new MapperConfiguration(cfg =>
{
SMPContext ctx = null; ///why not an extra property in conf?
cfg.CreateProjection<Client, Models.ViewModels.Client>()
.ForMember(d => d.DeviceCount, conf => conf.MapFrom(s => ctx.Entry(s).Collection(c => c.Devices).Query().Count()));
});
}
...
public async Task<IActionResult> Index()
{
var res = await
_context
.Clients.ProjectTo<SMP.Models.ViewModels.Client>(_configuration, new { ctx = _context })
.ToListAsync();
...
I was experimenting with this too:
public async Task<IActionResult> Index()
{
var configuration = new MapperConfiguration(cfg =>
{
cfg.CreateProjection<Client, Models.ViewModels.Client>()
.ForMember(d => d.DeviceCount, conf => conf.MapFrom(s => _context.Entry(s).Collection(c => c.Devices).Query().Count()));
});
var res = await
_context
.Clients.ProjectTo<SMP.Models.ViewModels.Client>(configuration)
.ToListAsync();
...
Either way, I get such an exception, which is quite understandable:
InvalidOperationException: The LINQ expression
'___context_0.Entry(NavigationTreeExpression Value:
EntityReference: Client Expression: c).Collection(c =>
c.Devices).Query() .Count()' could not be translated.
I have tried using Map instead of ProjectTo, but no luck either, as IMapper.Map().AfterMap passes the entire enumerable as parameter instead of per item.
Is there any neat solution?

Answer 1

It looks like there is a simple and neat solution with AutoMapper after all: CreateProjection is smart enough to create a performant SQL mapping in such a case:
builder.Services.AddAutoMapper(config =>
config.CreateProjection<SMP.Models.DataModels.Client, SMP.Models.ViewModels.Client>()
.ForMember(d => d.DeviceCount, o=> o.MapFrom(s => s.Devices.Count))
);
Using it is simple as well:
var res = await _context.Clients
.ProjectTo<Models.ViewModels.Client>(_mapper/*injected IMapper*/.ConfigurationProvider).ToListAsync();
The generated SQL query looks like this: