Engine isolated by multiple namespaces

I want to make an Engine that is isolated by two namespaces. That is,
let say for example I’d like to make an Engine whose classes all live
in:

Car::BMW

And thus, my models for example should be placed in:

app/models/car/bmw/

And my tables should be prefixed by for example:

car_bmw_

I tried to accomplish this by having this code in lib/car/bmw/engine.rb

module Car
module BMW
class Engine < ::Rails::Engine
isolate_namespace Car::BMW # This will call: engine_name ‘car_bmw’
end
end
end

With this code whenever I generate a model however, the model is placed
in:

app/models/car
And the table is prefixed by:

car_

The reason for this can be found in
Rails::Generators::NamedBase.namespaced_path:

def namespaced_path
@namespaced_path ||= namespace.name.split("::").map {|m| m.underscore
}[0]
end

Which, as you can see, takes only the first part of the namespace. Does
anyone know why this is? Am I not supposed to doubly namespace models
and controllers or is this a bug?