static void Main()
{
		var server = new MongoClient(new MongoUrl("mongodb://localhost:27017"));
		var db = server.GetDatabase("FooBar"); 
		var foos = db.GetCollection<FooNest>("Foos");
		
		foos.DeleteMany((x) => true);
		
		foos.InsertOne(
			new FooNest{ 
				Name = "Parent", 
				NestedCollection = new []{
					new FooNest { 
						Name = "Child"
					}
				}
			});
		
		var itms2 = foos.AsQueryable()
			.Select(top => top.NestedCollection
				.Select(child => new {ParentName = top.Name, child.Name}));
				
		Console.WriteLine("Items returned: " + itms2.ToList().ToJson());
		Console.WriteLine();
		Console.WriteLine("Query translation: " + itms2.ToString());
}

class FooNest
{
	public string Name;
	public IEnumerable<FooNest> NestedCollection;
}

Items returned: [[{ "ParentName" : "Child", "Name" : "Child" }]]

Query translation: aggregate([{ "$project" : { "__fld0" : { "$map" : { "input" : "$NestedCollection", "as" : "child", "in" : { "ParentName" : "$$child.Name", "Name" : "$$child.Name" } } }, "_id" : 0 } }])