Prepare MongoDB

Goal for this step

Decide where MongoDB will run, get it online, and capture the connection details in .env files so NestJS can read them.

Option A — Docker container (local database)

Install Docker Desktop if you have not already.
Create a Compose file at the project root:

cat <<'YAML' > docker-compose.dev.yml
version: "3.9"
services:
  mongodb:
    image: mongo:7
    container_name:
    ports:
      - "27017:27017"
    volumes:
      - mongo-data:/data/db
    environment:
      MONGO_INITDB_DATABASE: tasks
volumes:
  mongo-data:
YAML

Start MongoDB:

docker compose -f docker-compose.dev.yml up -d

Verify the container is running with docker ps or by connecting using mongosh "mongodb://localhost:27017" --eval 'db.runCommand({ ping: 1 })'.

When to choose Docker

Works offline once the image is pulled.
Mimics professional local development setups.
Easy to reset if you break the data—just stop and start the container.

Watch out

Docker Desktop needs plenty of disk space and at least 4 GB RAM to stay responsive.
Container terminology (ports, volumes) introduces new vocabulary; you can ignore it for now.

Learn Docker

Docker shows up in almost every modern backend workflow: local development, CI pipelines, and deployment all rely on containers.
Even light familiarity (building images, running docker compose) can get you a long way.
Set aside time later to explore the official Docker getting started guide so you understand what the commands here are doing under the hood.
A quick Docker 101 that is worth watching is here.

Option B — MongoDB Atlas (cloud database)

Create a free account at mongodb.com/cloud/atlas.
Create a new Project, then an M0 (Free Tier) cluster.
Add a database user (username + password) with Atlas admin or Read and write to any database privileges.
Add your current IP address to the network access list so Atlas accepts connections from your laptop.
Copy the Connection String for the driver MongoDB for VS Code or other and keep it handy—you will paste it into the .env file in a moment.

Why pick Atlas?

No heavy local installs—great for low-spec or school laptops.
Browser UI lets you browse collections to understand the data visually.
Mirrors how production cloud databases handle authentication and IP allow lists.

Atlas gotchas

Requires reliable internet; slow Wi-Fi makes requests feel sluggish.
In most cases, mobile tethering won't connect to Atlas.
Initial setup spans several screens (project, cluster, user, network access).
Connection strings are long—copy/paste carefully to avoid typo-driven auth errors.

Capture configuration in `.env` files

Create a template:

cat <<'ENV' > .env.example
# Shared defaults (keep secrets out of this file)
MONGODB_URI=mongodb://localhost:27017/tasks
MONGODB_URI_ATLAS=
USE_ATLAS=false
ENV

Create your personal local file (auto-ignored by git) and fill it out:

cp .env.example .env.development.local

If you are using Docker, the default MONGODB_URI already points to the container.
If you are using Atlas, set USE_ATLAS=true and paste the connection string into MONGODB_URI_ATLAS.

Keep secrets safe

Never commit .env.development.local. Confirm your .gitignore pattern includes .env* (the starter repo already does).

Ready for the next step

At this point you have:

A MongoDB instance running locally or an Atlas cluster accessible from your laptop.
Environment variables capturing the right connection details.

Continue to Connect MongoDB to install the NestJS database dependencies and wire the connection into AppModule.

Checkpoint: capture your changes in Git

Run git status and stage the files you created (for example, .env.example and docker-compose.dev.yml). Remember that personal files like .env.development.local should stay untracked.

git add .env.example docker-compose.dev.yml
git commit -m "feat: document local and atlas mongodb setup"

If you added Atlas notes or other docs, include them in the commit. You will push this commit to your feature branch once the whole chapter is complete.