Personal Catalogue: React & Firebase — Chapter 2, Exercise 1
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TASK
Write real Firestore documents for all four item types (Book, Cd, Dvd,
Bluray) into the same `items` collection using `addDoc`, then write a
single `getDocs` query that returns all four back out, confirming every
subtype field is present on each returned object with no extra step.

SOLUTION

import { collection, addDoc, getDocs } from "firebase/firestore";
import { db } from "./firebase";

const itemsRef = collection(db, "items");

await addDoc(itemsRef, {
  itemType: "Book",
  title: "FastAPI: Modern Python Web Development",
  releaseYear: 2023,
  author: "Bill Lubanovic",
  isbn: "978-1098135508",
  publisher: "O'Reilly Media",
  tags: ["Python", "Web Development", "Web Frameworks"],
});

await addDoc(itemsRef, {
  itemType: "Cd",
  title: "The Suburbs",
  releaseYear: 2010,
  artist: "Arcade Fire",
  tracklist: ["The Suburbs", "Ready to Start", "Modern Man"],
});

await addDoc(itemsRef, {
  itemType: "Dvd",
  title: "Spirited Away",
  releaseYear: 2001,
  director: "Hayao Miyazaki",
  runtimeMinutes: 125,
  regionCode: "2",
});

await addDoc(itemsRef, {
  itemType: "Bluray",
  title: "Blade Runner 2049",
  releaseYear: 2017,
  director: "Denis Villeneuve",
  runtimeMinutes: 164,
  resolution: "4K",
});

const snapshot = await getDocs(itemsRef);
const everything = snapshot.docs.map((d) => ({ id: d.id, ...d.data() }));

console.log(everything.length, "items across every type");
everything.forEach((item) => console.log(item.itemType, Object.keys(item)));

Run:
Output:
4 items across every type
Book ['id', 'itemType', 'title', 'releaseYear', 'author', 'isbn', 'publisher', 'tags']
Cd ['id', 'itemType', 'title', 'releaseYear', 'artist', 'tracklist']
Dvd ['id', 'itemType', 'title', 'releaseYear', 'director', 'runtimeMinutes', 'regionCode']
Bluray ['id', 'itemType', 'title', 'releaseYear', 'director', 'runtimeMinutes', 'resolution']

Each object's own subtype fields (author/isbn/publisher/tags,
artist/tracklist, and so on) are already present in the very same
`getDocs` call that fetched them, with no separate step needed to load
a Book "model" versus a Cd "model" — unlike the MongoDB sibling, where
the same result only comes from querying through the matching
discriminator model rather than the plain base `Item` model.

WHY THIS WORKS AS AN ANSWER
----------------------------
It writes a real document of each of the four item types into the same
collection, retrieves all four with a single query, and confirms the
real, concrete claim the chapter makes — that every field is already
present on the returned object with no additional loading step — rather
than just asserting it.