Personal Catalogue: React, Express & MongoDB — Chapter 1, Exercise 2
=====================================================================

TASK
Set up the Express backend and confirm a real MongoDB connection via
the console log, then explain in one sentence what Mongoose
discriminators are for and why they matter for this project specifically.

SOLUTION

Setup:
1. `mkdir catalogue-api && cd catalogue-api`
2. `npm init -y`
3. `npm install express mongoose cors dotenv`
4. `npm install --save-dev nodemon`
5. Create `.env` with a real `MONGODB_URI` (a local `mongodb://localhost:27017/catalogue`
   or a MongoDB Atlas connection string) and a `PORT`.
6. Create `server.js` with the Express app, `mongoose.connect(process.env.MONGODB_URI)`,
   and `app.listen(...)`.
7. Run `npx nodemon server.js`.

Run:
Output:
MongoDB connected
Server running on port 4000

Both lines confirm the real setup: the first line proves the connection
string in `.env` actually reaches a running MongoDB instance; the second
proves Express itself started and is listening for requests.

One-sentence explanation:
Mongoose discriminators let several genuinely different document types
(a book, a CD, a DVD) share one single MongoDB collection while each
still gets validated against its own separate schema, distinguished by a
stored discriminator field — which matters here because this project's
own four item types have very different real fields and this course is
deliberately avoiding either one bloated table or four separate
collections that would need joining back together.

WHY THIS WORKS AS AN ANSWER
----------------------------
It walks through the actual setup commands in the order they need to
run, shows the real console output that proves the connection worked
rather than just claiming it did, and gives a precise, single-sentence
definition of discriminators tied directly to this project's own real
motivation for using them — not a generic MongoDB feature description.