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

MongoError: failed to connect to server [mongodb:27017] on first connect

Do I have to add the containers into the same network to get external_links working?

MongoDB:

version: '2'
services:
  mongodb:
    image: mongo:3.4
    restart: always
    ports:
      - "27017:27017"
    volumes:
      - data:/data/db
volumes:
  data:

App:

version: '2'
services:
  app-dev:
    restart: Always
    build: repository/
    ports:
      - "3000:80"
    env_file:
      - ./environment.env
    external_links:
      - mongodb_mongodb_1:mongodb

Networks:

# sudo docker network ls
NETWORK ID          NAME                      DRIVER              SCOPE
29f8bae3e136        bridge                    bridge              local
67d5519cb2e6        dev_default               bridge              local
9e7097c844cf        host                      host                local
481ee4301f7c        mongodb_default           bridge              local
4275508449f6        none                      null                local
873a46298cd9        prod_default              bridge              local

Answer 1

Documentation at https://docs.docker.com/compose/compose-file/#/externallinks says

If you’re using the version 2 file format, the externally-created containers must be connected to at least one of the same networks as the service which is linking to them.

Ex:

Create a new docker network

docker network create -d bridge custom

docker-compose-1.yml

    version: '2'

    services:
      postgres:
        image: postgres:latest
        ports:
          - 5432:5432
        networks:
          - custom

    networks:
      custom:
        external: true

docker-compose-2.yml

    version: '2'

    services:
      app:
        image: training/webapp
        networks:
          - custom
        external_links:
          - postgres:postgres

    networks:
      custom:
        external: true

Answer 2

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:
      - <<network created by other compose file>>
    external_links:
      - postgres:postgres

networks:
  <<network created by other compose file>>:
    external: true