While running my docker-compose of a frontend, backend and mongo instance, the backend is not able to connect to the mongo db container.

I am using Mongoose and on NestJs on the backend, the connection code looks like this:

const mongoUser = process.env.MONGO_USER;
const mongoPassword = process.env.MONGO_PASSWORD;
const mongoHostname = process.env.MONGO_HOSTNAME;
const mongoPort = process.env.MONGO_PORT;


const url = `mongodb://${mongoUser}:${mongoPassword}@${mongoHostname}:${mongoPort}`;

@Module({
  imports: [MongooseModule.forRoot(url, {
    useNewUrlParser: true,
    "user": process.env.MONGO_USER,
    "pass": process.env.MONGO_PASSWORD....

And the corresponding docker-compose.yaml section looks like this:

  backend:
    container_name: backend
    build:
      context: ./backend
      dockerfile: Dockerfile
    ports:
      - 4001:4001
    depends_on:
      - mongo
    links:
      - mongo
    environment:
      MONGO_USER: root
      MONGO_PASSWORD: example
      MONGO_HOSTNAME: mongo
      MONGO_PORT: 27017
      PORT: 4001
    command: npm run start:dev

  mongo:
    image: mongo
    restart: always
    ports:
     - 27017:27017
    environment:
      MONGO_INITDB_ROOT_USERNAME: root
      MONGO_INITDB_ROOT_PASSWORD: example

In the logs of the container I can see a connection is being attempted, but the authentication fails.

Here are the logs. It's strange because the environment ports work and point it at the database, but apparently mongoDB is not recognizing the username and password.

backend          | [Nest] 34   - 01/05/2021, 9:09:49 PM   [InstanceLoader] JwtModule dependencies initialized +1ms
mongo_1          | {"t":{"$date":"2021-01-05T21:09:49.815+00:00"},"s":"I",  "c":"NETWORK",  "id":22943,   "ctx":"listener","msg":"Connection accepted","attr":{"remote":"172.18.0.5:47566","connectionId":4,"connectionCount":3}}
mongo_1          | {"t":{"$date":"2021-01-05T21:09:49.847+00:00"},"s":"I",  "c":"NETWORK",  "id":51800,   "ctx":"conn4","msg":"client metadata","attr":{"remote":"172.18.0.5:47566","client":"conn4","doc":{"driver":{"name":"nodejs|Mongoose","version":"3.6.3"},"os":{"type":"Linux","name":"linux","architecture":"x64","version":"4.19.76-linuxkit"},"platform":"'Node.js v14.15.4, LE (unified)","version":"3.6.3|5.11.9"}}}
mongo_1          | {"t":{"$date":"2021-01-05T21:09:49.856+00:00"},"s":"I",  "c":"NETWORK",  "id":22943,   "ctx":"listener","msg":"Connection accepted","attr":{"remote":"172.18.0.5:47568","connectionId":5,"connectionCount":4}}
mongo_1          | {"t":{"$date":"2021-01-05T21:09:49.876+00:00"},"s":"I",  "c":"NETWORK",  "id":51800,   "ctx":"conn5","msg":"client metadata","attr":{"remote":"172.18.0.5:47568","client":"conn5","doc":{"driver":{"name":"nodejs|Mongoose","version":"3.6.3"},"os":{"type":"Linux","name":"linux","architecture":"x64","version":"4.19.76-linuxkit"},"platform":"'Node.js v14.15.4, LE (unified)","version":"3.6.3|5.11.9"}}}
mongo_1          | {"t":{"$date":"2021-01-05T21:09:49.877+00:00"},"s":"I",  "c":"ACCESS",   "id":20251,   "ctx":"conn5","msg":"Supported SASL mechanisms requested for unknown user","attr":{"user":"undefined@admin"}}
mongo_1          | {"t":{"$date":"2021-01-05T21:09:49.877+00:00"},"s":"I",  "c":"ACCESS",   "id":20249,   "ctx":"conn5","msg":"Authentication failed","attr":{"mechanism":"SCRAM-SHA-256","principalName":"undefined","authenticationDatabase":"admin","client":"172.18.0.5:47568","result":"UserNotFound: Could not find user \"undefined\" for db \"admin\""}}
mongo_1          | {"t":{"$date":"2021-01-05T21:09:49.880+00:00"},"s":"I",  "c":"ACCESS",   "id":20249,   "ctx":"conn5","msg":"Authentication failed","attr":{"mechanism":"SCRAM-SHA-1","principalName":"undefined","authenticationDatabase":"admin","client":"172.18.0.5:47568","result":"UserNotFound: Could not find user \"undefined\" for db \"admin\""}}
mongo_1          | {"t":{"$date":"2021-01-05T21:09:49.883+00:00"},"s":"I",  "c":"NETWORK",  "id":22944,   "ctx":"conn4","msg":"Connection ended","attr":{"remote":"172.18.0.5:47566","connectionId":4,"connectionCount":3}}
backend          | [Nest] 34   - 01/05/2021, 9:09:49 PM   [MongooseModule] Unable to connect to the database. Retrying (1)... +73ms

I beautified the JSON output from mongo and I found UserNotFound: Could not find user \"undefined\" for db \"admin\":

{
  "t": {
    "$date": "2021-01-05T21:09:49.877+00:00"
  },
  "s": "I",
  "c": "ACCESS",
  "id": 20251,
  "ctx": "conn5",
  "msg": "Supported SASL mechanisms requested for unknown user",
  "attr": {
    "user": "undefined@admin"
  }
} {
  "t": {
    "$date": "2021-01-05T21:09:49.877+00:00"
  },
  "s": "I",
  "c": "ACCESS",
  "id": 20249,
  "ctx": "conn5",
  "msg": "Authentication failed",
  "attr": {
    "mechanism": "SCRAM-SHA-256",
    "principalName": "undefined",
    "authenticationDatabase": "admin",
    "client": "172.18.0.5:47568",
    "result": "UserNotFound: Could not find user \"undefined\" for db \"admin\""
  }
}

According to the documentation the database name should be at the end of the url like:

url = mongodb://${mongoUser}:${mongoPassword}@${mongoHostname}:${mongoPort}/DB-NAME-HERE

Since you can use the options "user", "pass" and "dbName", I would suggest to prune from the url of the credentials or remove the options user and pass.