Premier League Predictor: FastAPI & PostgreSQL — Chapter 4, Exercise 3
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TASK
Build the full click-to-pair page yourself against a real season and
gameweek, enter all 10 fixtures for a gameweek, and then write one
sentence explaining what happens when you try to click an 11th team
pairing that reuses an already-fixtured team.

SOLUTION

Steps to complete this exercise:

1. Make sure a Season with exactly 20 SeasonTeam rows exists (built in
   Chapter 3), and create a Gameweek for it via
   POST /api/seasons/{season_id}/gameweeks.

2. Save fixtures.html and fixtures.js from this chapter into the
   static/ folder from Chapter 1, updating the hardcoded seasonId and
   gameweekId constants at the top of fixtures.js to match the real
   season and gameweek just created.

3. Start the app (uvicorn main:app --reload) and open
   http://localhost:8000/fixtures.html.

4. Click through all 20 team buttons in pairs — 10 fixtures total —
   confirming after each Add Fixture click that both teams' own
   buttons become disabled (grayed out) and the Home/Away slots reset
   for the next pair.

5. Once all 20 teams have been used, every team button on the page
   should be disabled — there's nothing left to click, since the
   gameweek already has its full 10 fixtures.

To actually observe the exercise's own real question, reload the page
first (clearing usedTeamIds back to empty) so every button re-enables,
then try clicking two teams that were already used in an earlier
fixture and click Add Fixture.

One-sentence answer for the write-up: clicking Add Fixture with an
already-used team sends the request to the server as normal, but
create_fixture's own database-backed check rejects it with a 409 and
the message "One of these teams is already fixtured this gameweek,"
which the page displays via alert() — no duplicate fixture gets
created, confirming the client-side button graying is just a
convenience and the server's own check is what actually enforces the
rule.

WHY THIS WORKS AS AN ANSWER
----------------------------
It walks through the real setup and full 10-fixture entry process step
by step, then deliberately reproduces the "stale page" scenario from
Exercise 2 by reloading before attempting the duplicate, and closes
with a one-sentence answer that correctly names the real 409 response
and message rather than a vague "it doesn't work."