Docker-Compose Django can't connect to DB or Celery

Question

I have everything on one network (it talks to another app in another docker-compose file).

But I keep getting this error:

    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
    django.db.utils.OperationalError: could not connect to server: Connection refused
    Is the server running on host "db" (192.168.208.3) and accepting
    TCP/IP connections on port 5000?

When I change my SQL_PORT=5432 (default port that's running in the postgres container) the error above disappears and my app is up, but then it has problems when trying to connect to celery or in shell it says db is not connected.

I have to use 5000 cause there is another postgres db on the other app in the second docker-compose setup.

I think I'm lost on the internals of networks. I was pretty sure I am suppose to use the exposed port of 5000 for my database.

version: "3.9"
   
services:
  app:
    build: .
    command: python manage.py runserver 0.0.0.0:8000
    container_name: app
    environment:
      - DEBUG=True
      - PYTHONUNBUFFERED=1
      - CELERY_BROKER=redis://broker:6379/0
      - CELERY_BACKEND=redis://broker:6379/   
      - APP_BASIC_AUTH_PASSWORD=adPswd12*
      - APP_BASIC_AUTH_USER=admin
      - APP_TOKEN_AUTH=NHEC_UTILITY
      - VTN_API_URL=vtn_app:8000
      - VTN_API_TOKEN=NHECAPP_UTILITY
      - SQL_PORT=5000
    volumes:
      - .:/code
    ports:
      - "9000:8000"
    networks:
      - app-web-net  
    depends_on:
      - db
      - celery-worker
      - broker
  app_test:
    build: .
    command: python manage.py test
    container_name: app_test
    environment:
      - DEBUG=True
      - PYTHONUNBUFFERED=1
    volumes:
      - .:/code
    depends_on:
      - db
  db:
    image: postgres:10
    container_name: app_postgres
    ports:
      - 5000:5432
    volumes:
      - db_data:/var/lib/postgresql/data
    environment:
      POSTGRES_DB: nhec
      POSTGRES_USER: nhec
      POSTGRES_PASSWORD: nhec
    networks:
      - app-web-net
  celery-worker:
    build: .
    command: celery -A app worker -l DEBUG
    depends_on:
      - db
      - broker
    environment:
      CELERY_BROKER_URL: redis://broker:6379/0
    networks:
      - app-web-net
  broker:
    image: redis:6-alpine
    ports:
      - 6379:6379
    networks:
      - app-web-net

volumes: 
  db_data: {}

networks: 
  app-web-net:
    driver: bridge

Answer 1

The ports will expose the postgres server on the your localhost port 5000, not to internal services. Your services will be able to reach the database container and its ports within the same network.

If you still want to change the default postgres port, here's a related answer that you might find helpful. Changing a postgres containers server port in Docker Compose

Your container name is actually app_postgres, not db as specified by this in your docker compose:

container_name: app_postgres

The other thing you can do is change the name of the container from app_postgres to something not the same as the postgres in your other app's docker compose file. Both postgres instances can have port 5432 exposed for your apps to use.