Investigating OData's internals

Hey guys,

I'm often in a situation where I need things resolved quickly.
To this end I'm trying to learn more about the inner workings of the OData framework in the hopes that one day I might feel confident to submit the odd pull request.

I've hit a bit of a block in my understanding of the process involved here and was wondering if I could get a hint or two to help me resolve the block.

My original problem started here (https://github.com/OData/AspNetCoreOData/issues/248).
I have since pulled down the OData code from here and added it to my solution in order to investigate the problem by stepping through it in my solution.

To cut a long story short here I post the App objects without the Roles as a single array post to a custom action with an OData query asking ?$expand=Item($expand=Roles) so i'm hoping to get all this back and that's not happening.

So my response object is an IQueryable<Result<App>> that looks like this (I've omitted other properties not relevant to the problem) ...

var result = new [] 
{
	new Result<App> {
		Success = "True",
		Message = "Created Successfully",
		Item = new App { 
			Id = 1, 
			DefaultCultureId = "", 
			Name = "TestApp1", 
			Domain = "Domain1", 
			DefaultTheme = "Default", 
			Roles: [
				new Role { Id = Guid.NewGuid(), Name = "Administrators" },
				new Role { Id = Guid.NewGuid(), Name = "Users" },
				new Role { Id = Guid.NewGuid(), Name = "Guests" }
			] 
		 }
	},
	new Result<App> {
		Success = "True",
		Message = "Created Successfully",
		Item = new App { 
			Id = 2, 
			DefaultCultureId = "", 
			Name = "TestApp2", 
			Domain = "Domain2", 
			DefaultTheme = "Default", 
			Roles: [
				new Role { Id = Guid.NewGuid(), Name = "Administrators"	},
				new Role { Id = Guid.NewGuid(),	Name = "Users" },
				new Role { Id = Guid.NewGuid(), Name = "Guests" }
			] 
		 }
	}
};

Digging through the execution as it's running reveals that the EnableAttribute when applied to my Iqueryable returns an expression tree that looks like this ...

System.Collections.Generic.List`1[Core.Objects.Result`1[Core.Objects.Entities.CMS.App]]
    .Select($it => 
		new SelectAllAndExpand`1() {
			Model = value(Microsoft.AspNetCore.OData.Query.Container.LinqParameterContainer+TypedLinqParameterContainer`1[Microsoft.OData.Edm.IEdmModel]).TypedProperty, 
			Instance = $it, 
			UseInstanceForProperties = True, 
			Container = new SingleExpandedProperty`1() 
			{
				Name = "Item", 
				Value = new SelectAllAndExpand`1() 
				{
					Model = value(Microsoft.AspNetCore.OData.Query.Container.LinqParameterContainer+TypedLinqParameterContainer`1[Microsoft.OData.Edm.IEdmModel]).TypedProperty, 
					Instance = IIF(($it == null), null, $it.Item), 
					UseInstanceForProperties = True, 
					Container = new NamedProperty`1() 
					{
						Name = "Roles", 
						Value = IIF(
							(IIF((IIF(($it == null), null, $it.Item) == null), null, IIF(($it == null), null, $it.Item).Roles) == null), null, IIF((IIF(($it == null), null, $it.Item) == null), null, IIF(($it == null), null, $it.Item).Roles)
								.Select($it => new SelectAll`1() 
								{
									Model = value(Microsoft.AspNetCore.OData.Query.Container.LinqParameterContainer+TypedLinqParameterContainer`1[Microsoft.OData.Edm.IEdmModel]).TypedProperty, 
									Instance = $it, 
									UseInstanceForProperties = True
								})
						)
					}
				}, 
				IsNull = (IIF(($it == null), null, $it.Item) == null)
			}
		}
	)

My confusion here is why did this not get translated in to something like this ...

result.Select(i => new {
	// all scalar props
	// odata props if any like etags or meta links
	Roles = i.Roles?.Select(j => {
		// all scalar props
		// odata props if any like etags or meta links
	})
})

Instead it seems that these wrappers have been introduced.
Would a direct LINQ expression tree not make the whole process easier to maintain and then from a serialization point of view we could simply do ...

await repsonse.WriteAsync(result.ToJson(serializationOptions));

I have most likely missed something here that the framework is doing but I figured I would ask as this seems like the next logical improvement to simplify the codebase in my understanding right now.

Start with issue #248, then trace the EnableAttribute path that produces the SelectAllAndExpand and NamedProperty expression wrappers. Compare that behavior with the proposed direct LINQ shape and review the surrounding OData serialization flow; done would require a decided, documented scope for any internal simplification.