How to configure NixOS with nvidia on an Optimus laptop?

I have an Acer Aspire E15 laptop model E5-57G-38HC with integrated intel graphics card and a nvidia GeForce 840M card. I enabled bumblebee in my configuration.nix but only the intel card is working and when i look at the list of modules, the nvidia module is missing. When installing bumblebee i noticed that the installation process downloaded the nvidia driver but for some reason the module does not load.

This is my configuration.nix: http://pastebin.com/2kepcGfr

and my hardware-configuration.nix: http://pastebin.com/57wCmzN8

What am I doing wrong ?