Connection Error on Node.js App with Fly.io Postgres database

TLDR;
I want to connect to on Node.js(express).
hostname “.internal” or “.fly.dev” didn’t work at all.
How can i solve this?

Hello, i’m new to Fly.io and Postgres.
I deployed my app few days ago, and it worked successfully.

My App is based on Node.js (express), Socket.io, and I’m trying to use Postgres Database with my app.
But, i don’t know how to do this.

I tried to add codes of Node.js connection code with Connection String, and also with option objects like
https://fly.io/docs/postgres/connecting/app-connection-examples/
, but it didn’t work. There was an error.

listening on port*: 7777
node:internal/process/promises:265
            triggerUncaughtException(err, true /* fromPromise */);
            ^

Error: getaddrinfo ENOTFOUND <my-app-db>.internal
    at GetAddrInfoReqWrap.onlookup [as oncomplete] (node:dns:71:26) {
  errno: -3008,
  code: 'ENOTFOUND',
  syscall: 'getaddrinfo',
  hostname: '<my-app>.internal'
}

I think the problem is hostname. So i’ve changed it with ‘.fly.dev’, which i can see on my dashboard. But there was an another error.

const error = this._ending ? new Error('Connection terminated') : new Error('Connection terminated unexpectedly')
                                                                  ^
                             Connection terminated unexpectedly')
Error: Connection terminated unexpectedly
    at Connection.<anonymous>

When i create Postgres cluster, the log was:

  Username: postgres
  Password: <password>
  Hostname: <my-app-db>.internal
  Proxy port: 5432
  Postgres port: 5433
  Connection string: postgres://postgres:<password>@<my-app-db>.internal:5432

I tried another hostname with .fly.dev, but it didn’t work too.
Neither of local or deployed app doesn’t work.

How can i use with ?
My mind is almost blown up because of this, all this week.
Please help me. Thanks.