mongo-express | Could not connect to database using connectionString: mongodb://127.0.0.1:27017/" - mongodb

Question

When trying to spin up a docker container with docker-compose,
I get the following error message:
(node:8) [MONGODB DRIVER] Warning: Current Server Discovery
and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
mongo-express | Could not connect to database using connectionString: mongodb://127.0.0.1:27017/"
I noticed that mongo-express always adds a double quote to the end of my connection string, is this intentional or did I mess something up in my docker-compose file?
The file:
version: '3.8'
services:
mongodb:
image: mongo:latest
container_name: mongodb
restart: unless-stopped
networks:
- backbone
expose:
- 27017
ports:
- 27017:27017
env_file:
- ./.env
command: [--auth]
environment:
- MONGO_INITDB_ROOT_USERNAME=${DB_USERNAME}
- MONGO_INITDB_ROOT_PASSWORD=${DB_PASSWORD}
volumes:
- ./data:/data/db
mongo-express:
image: mongo-express
container_name: mongo-express
restart: unless-stopped
networks:
- backbone
ports:
- 8081:8081
environment:
- ME_CONFIG_MONGODB_SERVER=127.0.0.1
- ME_CONFIG_MONGODB_URL="mongodb://127.0.0.1:27017"
- ME_CONFIG_BASICAUTH_USERNAME=${DB_USERNAME}
- ME_CONFIG_BASICAUTH_PASSWORD=${DB_PASSWORD}
networks:
backbone:
driver: bridge
I already tried updating my version of docker/docker-compose and mongodb.
How do I resolve the error?
Or is there a workaround I could use?

Answer 1

In a container, localhost refers to the container itself. So when Mongo Express tries to connect to the database at localhost, it looks for the database inside the Mongo Express container.
Docker compose let's you refer to containers using their container names, so you should change your Mongo Express environment variables to
- ME_CONFIG_MONGODB_SERVER=mongodb
- ME_CONFIG_MONGODB_URL="mongodb://mongodb:27017"