# Period selector missing older months — diagnosis

## Context

User reports the "From / To" period selector on the **adults** dashboard now
shows **Dec 2025** as the oldest available period. The older production
deployment shows Sep+Oct 2025, Nov 2025, Dec 2025+Jan 2026 (merged labels) —
i.e. data going back to September 2025. The user wants to know what went
sideways. Confirmed: the dropdown options on both Python and Go genuinely
start at Dec 2025, not just the default selection — Sep/Oct/Nov 2025 are not
in the list at all.

## Root cause — the live adults sheet header is missing those columns

The fresh cache files at `tmp/go/attendance_regular_cache.json` (raw rows from
Google Sheets, modifiedTime `2026-05-06T22:30:02Z`, cached `2026-05-08T00:26`)
contain the actual header row for the adults tab (gid=0):

```text
['FUJ tréninky úterý 20:30-22:00', '', '', '02.12.2025', '09.12.2025',
 '16.12.2025', '06.01.2026', '13.01.2026', '20.01.2026', '27.01.2026',
 '03.02.2026', '10.02.2026', '17.02.2026', '24.02.2026', '03.03.2026',
 '10.03.2026', '17.03.2026', '23.03.2026', '31.03.2026', '13.04.2026',
 '20.04.2026', '27.04.2026', '04.05.2026', '', '', '']
```

**The first date column in the live adults sheet is `02.12.2025` (Dec 2 2025).**
There are no September, October, or November 2025 columns in the header at
all. Both backends parse this faithfully (no slicing, no cutoff anywhere) and
correctly produce `sortedMonths = ["2025-12", "2026-01", …, "2026-05"]`.

The juniors sheet (different tab, `JUNIOR_SHEET_GID`) is **fine** — its header
still contains `['', 'tier', '', '15.09.2025', '13.10.2025', '20.10.2025',
'03.11.2025', '24.11.2025', '10.11.2025', '17.11.2025', '01.12.2025', …]`. So
the juniors page still shows Sep+Oct / Nov / Dec+Jan correctly.

So this is a **data issue in the adults attendance Google Sheet**: at some
point between when production's cache was last warmed (showing Sep–Nov) and
2026-05-06, somebody (or some action) removed the columns for September,
October, and November 2025 from the adults tab header.

The code is doing exactly what it should. There is no parser regression.

## What to do

### 1. Restore the missing date columns in the adults attendance sheet

The fix lives in Google Sheets, not in the codebase. Options, in order of
preference:

- **(a) Use Sheets version history.** File → Version history → See version
  history; find a version from before the columns were dropped (anything
  before about Mar 2026 should still have them). Copy the Sep/Oct/Nov 2025
  date column headers and the `TRUE/FALSE` cells underneath them back into
  the current sheet. Only restore the 11 missing date columns; do not
  full-revert (you'd lose every change since then).
- **(b) Pull from the production server's cache.** The production deployment
  evidently still has the older cache, since its dashboard renders those
  months. SSH there, copy `tmp/attendance_regular_cache.json`, and you can
  reconstruct the per-member Sep/Oct/Nov attendance counts from the
  `data[*][2]` map (keys `"2025-09"`, `"2025-10"`, `"2025-11"`). Re-enter
  those into the sheet manually as date columns + `TRUE` cells — tedious but
  deterministic.
- **(c) Accept the loss.** If the older columns aren't recoverable, the
  dashboard correctly reflects what the sheet contains; nothing more to do.

Which to pick depends on whether those months still need to be billed /
reconciled.

### 2. Restore `ADULT_MERGED_MONTHS` (user confirmed this was unintentional)

Independent of the sheet issue: commit `1257f0d` (Mar 9 2026) commented out
the adult merge mappings. Once the Sep/Oct/Nov columns are back in the sheet,
the dashboard would still show them as separate periods instead of the
production-style "Sep+Oct 2025" and "Dec 2025+Jan 2026" merged labels.

User confirmed this was unintentional. Two files to update:

- [scripts/attendance.py:32-35](scripts/attendance.py#L32-L35) — uncomment
  the two mappings:

  ```python
  ADULT_MERGED_MONTHS = {
      "2025-12": "2026-01",   # keys are merged into values
      "2025-09": "2025-10",
  }
  ```

- [go/internal/services/membership/sources.go:30](go/internal/services/membership/sources.go#L30) — mirror the same:

  ```go
  var AdultMergedMonths = map[string]string{
      "2025-12": "2026-01",
      "2025-09": "2025-10",
  }
  ```

After this change, hit `POST /flush-cache` on each backend so the in-process
post-processed adults cache is rebuilt with the new mapping.

### 3. (Optional, separate) Fix the JS auto-default that hides older months

This is **not** the cause of the user's current symptom (that's the sheet
issue), but it will become a UX issue once Sep/Oct/Nov columns are restored:
the Python frontend's `defaultFrom = Math.max(0, maxMonthIdx - 4)` will still
default the From-selector to ~5 months before the latest column on every page
load, hiding restored older months until the user manually picks them.

- [templates/adults.html:1047](templates/adults.html#L1047) — `var defaultFrom = Math.max(0, maxMonthIdx - 4);`
- [templates/juniors.html:1028](templates/juniors.html#L1028) — same line.

Drop those four lines (`defaultFrom`, `fromSelect.value = defaultFrom`,
`toSelect.value = maxMonthIdx`, `applyMonthFilter()`) so the page loads with
all non-future months visible — matching the Go side, which only calls
`hideFutureMonths()` and leaves From at its first option.

Recommend bundling this with step 2 since they touch related UI.

## Verification

1. **After step 1** — `POST /flush-cache` on Python and Go backends; reload
   `/adults` on each. Confirm the dropdown now lists Sep/Oct/Nov 2025.
2. **After step 2** — reload `/adults`. Confirm the dropdown shows
   "Sep+Oct 2025" as a single period and "Dec 2025+Jan 2026" as a single
   period. (Still requires the sheet columns to exist.)
3. **After step 3** — reload `/adults` and `/juniors` on Python. Confirm the
   table renders all non-future months on first load (Sep 2025 through the
   current month) instead of starting at Dec 2025.
4. **Parity check** — `make parity` should report zero diffs between Python
   and Go on `/api/adults` and `/api/juniors`.

## Critical files referenced

- `tmp/go/attendance_regular_cache.json` — current adults sheet rows
  (evidence: header starts at `02.12.2025`).
- `tmp/go/attendance_juniors_cache.json` — current juniors sheet rows
  (header still has `15.09.2025`).
- [scripts/attendance.py](scripts/attendance.py) — `ADULT_MERGED_MONTHS`
  empty after `1257f0d`; `parse_dates` / `group_by_month` faithful.
- [go/internal/services/membership/sources.go](go/internal/services/membership/sources.go) — Go counterpart, same shape.
- [templates/adults.html](templates/adults.html), [templates/juniors.html](templates/juniors.html) — JS onload `defaultFrom = -4` issue (step 3).
- [go/internal/web/static/js/filters.js](go/internal/web/static/js/filters.js) — Go filter UI (already correct, no changes).