Premier League Predictor: Django & MySQL — Chapter 10, Exercise 3
====================================================

TASK
Build the selector into a real setup with at least two seasons and two
gameweeks, enter a fixture in one gameweek, navigate to the other
gameweek and back using only the selector dropdowns, and confirm the
previously-entered fixture's own two teams are still shown disabled —
then explain specifically why this works even though no client-side
state was ever preserved across the navigation at all.

SOLUTION

Using season 1 (2026/27) with gameweek_id 5 and gameweek_id 6, both
already existing:

1. Visit /gameweeks/5/fixtures/. The team grid renders with every
   button enabled, since gameweek 5 has no fixtures yet.

2. Click two teams to select them (say Arsenal and Chelsea) and click
   "Add Fixture." create_fixture creates a real Fixture row for
   gameweek 5, and addFixture()'s own JS updates usedTeamIds and calls
   disableUsedButtons(), so both buttons show disabled immediately on
   this same page, without a reload.

3. Use the gameweek selector dropdown (not the browser's back button,
   not a manual URL edit) to switch to Gameweek 6. This triggers
   location.href = '/gameweeks/6/fixtures/' — a genuine full page
   navigation, not an in-page update.

4. On the new page load for gameweek 6, fixture_entry runs completely
   fresh: it queries Fixture.objects.filter(gameweek=<gameweek 6>),
   which is empty, so used_team_ids is an empty set and every button
   renders without the disabled attribute. The page for gameweek 6
   correctly shows all 20 teams available, as expected — it has no
   memory of gameweek 5's own fixture at all, because it's a
   completely separate page render.

5. Use the gameweek selector again to switch back to Gameweek 5:
   location.href = '/gameweeks/5/fixtures/' loads yet another fresh
   page. fixture_entry runs again, queries
   Fixture.objects.filter(gameweek=<gameweek 5>) again, finds the real
   fixture created in step 2, and computes used_team_ids = {Arsenal's
   id, Chelsea's id} again — correctly, because it's reading the real
   row from the database, not any leftover in-memory JavaScript state.

6. The rendered HTML for this reload of gameweek 5 shows both
   Arsenal's and Chelsea's buttons with the disabled attribute already
   present, and fixtures.js's own opening line —
   new Set(Array.from(document.querySelectorAll('#team-grid button[disabled]'))...)
   — seeds usedTeamIds correctly from those already-disabled buttons
   the instant the page loads.

Why this works with zero preserved client-side state: every single
navigation in this sequence is a full page load, and fixture_entry
recomputes used_team_ids from the real Fixture table on every single
one of those loads. There's nothing to "lose" between gameweek 5 and
gameweek 6, because nothing was ever being remembered across pages in
the first place — the correctness comes entirely from the server
re-deriving the truth fresh each time, not from any state surviving
the trip.

WHY THIS WORKS AS AN ANSWER
----------------------------
It walks through a real sequence using genuine page navigations (not
just describing the concept), shows the disabled state correctly
disappearing on the empty gameweek and correctly reappearing on the
return to the fixtured one, and explicitly explains that the mechanism
is fresh server-side recomputation on every load rather than any
preserved client-side memory — the actual distinction the exercise
is testing for.