Accessing packages from multiple channels using flakes

Help
1

Hi, I’m trying to using flakes with my system configuration, but I’m struggling with importing packages from different branches of nixpkgs. At the moment, I have this :

{ inputs, ... }:
{
  environment.systemPackages = [ inputs.unstable.neofetch ];
}

but it seems that I cannot access unstable this way, or at least I cannot use this that way. Is there some kind of workaround to get over this ? I tried looking at Bqv’s configuration repository, but I cannot fully understand it at the moment, although it seems that he creates an overlay on pkgs attribute to use pkgs.channel.package. Does anyone has an idea ? Thanks for taking the time to read / answer.

2

using something like

let
  overlay-unstable = final: prev: {
    unstable = *name-of-the-unstable-input*.legacyPackages.${system};
  };
in nixpkgs.overlays = [ overlay-unstable.unstable ];

worked for the system configuration, however it does not seem to work for my HM configuration, based on Ryantm template, I tried something like

    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = stable.legacyPackages.${system};
        overlay-unstable = final: prev: {
          unstable = nixpkgs-unstable.legacyPackages.${system};
        };
      in {
        devShell = pkgs.mkShell rec {
          name = "home-manager-shell";

          buildInputs = with pkgs;
            [ (import home-manager { inherit pkgs; }).home-manager ];

          shellHook = ''
            export NIX_PATH="nixpkgs=${stable}:home-manager=${home-manager}"
            export HOME_MANAGER_CONFIG="./home.nix"
            PS1="${name}> "
          '';
        };
        nixpkgs.overlays = [ overlay-unstable.unstable ];
      });

but the overlay isn’t enabled, or at least I cannot use pkgs.unstable.package.