tl;dr: When setting up MongoDB with Docker Compose, make sure to specify the authSource=admin query parameter in the connection string if you are using the root account for authentication. This will allow you to access the database without getting an "Authentication failed" error.

Background

I set up MongoDB with Docker Compose. I used the MONGO_INITDB_ROOT_USERNAME and MONGO_INITDB_ROOT_PASSWORD environment variables to set the root user's username and password.

services:
  mongo:
    image: mongo
    restart: always
    environment:
      MONGO_INITDB_ROOT_USERNAME: root
      MONGO_INITDB_ROOT_PASSWORD: example
    ports:
      - 127.0.0.1:27017:27017
    volumes:
      - ./data/db:/data/db

Problem

I set my MONGODB_URI environment variable in my .env file:

MONGODB_URI=mongodb://root:example@localhost:27017/myproject

However, when accessing the database, I got the following error:

$ mongosh $MONGODB_URI
MongoServerError: Authentication failed.

Why it happened

It turns out, the root account only works when using the admin database as the authentication database.

Solution

To fix this, I need to specify the authSource query parameter in the connection string:

MONGODB_URI=mongodb://root:example@localhost:27017/myproject?authSource=admin