Docker-compose external_links not able to connect

Question

I have a couple of app containers that I want to connect to the mongodb container. I tried with external_links but I can not connect to the mongodb.

I get

Answer 1

Yuva's answer above for the version 2 holds good for version 3 as well.

The documentation for the external_links isn't clear enough.

For more clarity I pasted the version 3 variation with annotation

version: '3'

services:
  app:
    image: training/webapp
    networks:
      - <>
    external_links:
      - postgres:postgres

networks:
  <>:
    external: true