gistfile1.txt

    systemd.services.mypackage = {

      wantedBy = [ "multi-user.target" ];
      requires = [ "network.target" ]
        ++ optional useLocalPostgresqlDatabase "postgresql.service";
      after = [ "network.target" ]
        ++ optional useLocalPostgresqlDatabase "postgresql.service";
      before = [ "nginx.service" ];

      serviceConfig = {
        User = cfg.user;
        Group = cfg.group;
        WorkingDirectory = cfg.stateDir;
        ExecStart = "${pkgs.mypackage}/bin/mypackage";
        Restart = "always";
        PremissionsStartOnly = true;
        PrivateDevices = true;
        PrivateTmp = mkIf (!useLocalPostgresqlDatabase) true;
        JoinsNamespaceOf = mkIf useLocalPostgresqlDatabase "postgresql.service";
      };

      preStart = ''
        set -x
        ${optionalString useLocalPostgresqlDatabase ''
          if ! [ -e ${cfg.stateDir}/.db-created ]; then
            ${pkgs.postgresql}/bin/createuser ${cfg.database.user}
            ${pkgs.postgresql}/bin/createdb ${cfg.database.name} \
              -O ${cfg.database.user}
            touch ${cfg.stateDir}/.db-created
          fi
        ''}
      '';
    };