Express container unable to connect to Mongo Image with Docker-Compose - mongodb

Question

I am trying to connect an express container to MongoDB image with docker compose but the connection is being rejected, I can connect to the db with robomongo. I can't get what is happening, this is the express code that connects it:
mongoose.connect('mongodb://localhost:27017/database')
.then(()=>console.log('connection succesfull to url'))
.catch((err)=>console.error(err));
This is the docker-compose file
version: "3"
services:
backend:
build:
context: ../backend
dockerfile: ${PWD}/images/backend/Dockerfile
container_name: backend
ports:
- "${BACKEND_PORT}:${BACKEND_PORT}"
env_file:
- ./deploy.env
environment:
- PORT=3000
- MONGO_CONNECTION=${MONGO_CONNECTION}
command: npm start
links:
- mongodb
depends_on:
- mongodb
front-app:
build:
context: ../front-app
dockerfile: ${PWD}/images/angular/Dockerfile
container_name: front-app
ports:
- "${FRONTEND_PORT}:4200"
env_file:
- ./deploy.env
command: npm start
mongodb:
image: mongo:3.6
container_name: mongo
volumes:
- "${MONGO_DB_DATA}:/data/db"
- "${MONGO_DB_DATA}:/data/configdb"
ports:
- "27017:27017"
This is the error
backend | { MongoNetworkError: failed to connect to server [localhost:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017]
backend | at Pool.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/topologies/server.js:564:11)
backend | at Pool.emit (events.js:182:13)
backend | at Pool.EventEmitter.emit (domain.js:442:20)
backend | at Connection.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/connection/pool.js:317:12)
backend | at Object.onceWrapper (events.js:273:13)
backend | at Connection.emit (events.js:182:13)
backend | at Connection.EventEmitter.emit (domain.js:442:20)
backend | at Socket.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/connection/connection.js:246:50)
backend | at Object.onceWrapper (events.js:273:13)
backend | at Socket.emit (events.js:182:13)
backend | at Socket.EventEmitter.emit (domain.js:442:20)
backend | at emitErrorNT (internal/streams/destroy.js:82:8)
backend | at emitErrorAndCloseNT (internal/streams/destroy.js:50:3)
backend | at process._tickCallback (internal/process/next_tick.js:63:19)
backend | name: 'MongoNetworkError',
backend | errorLabels: [ 'TransientTransactionError' ],
backend | [Symbol(mongoErrorContextSymbol)]: {} }

Answer 1

Every process running in docker container thinks that he is "the only one in the world". It means that for this process localhost means: my, container's localhost. And your backend is alone in his container so that's why he cannot find mongodb under localhost.
To fix that problem you should place hostname "mongodb" instead of "localhost" as in docker-compose you can access services using theirs names - it means that mongodb container can also access your backend using "backend" domain.
Please note also that "links" is deprecated in docker and shouldn't be used - it's not required in your configuration as docker-compose gives every service within docker-compose file access to each other using method mentioned above.

Answer 2

Jakub Bujny answer is spot on, I did not understand because it did not include an example. So I am adding one.
mongoose.connect('mongodb://mongodb:27017/database')
.then(()=>console.log('connection succesfull to url'))
.catch((err)=>console.error(err));