Very simple elegant approach I am using:

The agenda is, to use stable Nix channel, except for packages we want explicitly to use from the other channels.

```

# nix-channel --add https://nixos.org/channels/nixos-unstable unstable

# nix-channel --update unstable

```


We don't swap `nixos`, so it stays defacto the channel for Nix to upgrade.

```

# nix-channel --list

nixos https://nixos.org/channels/nixos-17.09

unstable https://nixos.org/channels/nixos-unstable

```


Both channels are in the path already:

```

$ echo "$NIX_PATH"

nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels

```


And they are in:

```

$ ls /nix/var/nix/profiles/per-user/root/channels/

...  nixos  unstable

```


So their namespaces are seen to Nix through variable.


Now, in relevant `/etc/nixos/*.nix`:

```

{ config, pkgs, ... }:


let

  unstable = import <unstable> {};

in

{

  ...

  environment.systemPackages = with pkgs; [

    ...

    # Unstable packages

    ## fwupd package

    unstable.fwupd

    ...

  ];

}

```