just part of a bundleEnv

partial_default.nix

mongo = attrs: rec {
      buildInputs = [ zlib python27Packages.python python27Packages.pymongo mongodb ];
      _check = "${ruby_2_0.gemPath}/gems/sensu-plugins-mongodb-${(import ./gemset.nix).sensu-plugins-mongodb.version}/bin/check-mongodb.py";
      postInstall = ''
        # the ruby check runs a python script
        patchShebangs $out/${_check}
      '';
    };