BE: Fix: NIC-Covered Parent Devices Miss Connected/Down Reconnected Events #1821

.github/skills/scan-pipeline/SKILL.md

@@ -56,6 +56,7 @@ This is the scan-pipeline-local half of a bigger attribution system — see `dat
 5. **A blank/null-equivalent `scanMac` can create a phantom `Devices` row.** `create_new_devices()`'s two creation-path queries filter `scanMac NOT IN (NULL_EQUIVALENTS_SQL)` (`server/scan/device_handling.py`, `const.NULL_EQUIVALENTS_SQL`) as a backstop, because `scanCreatesDevice` defaults to `1` — any plugin reporting a row with no real MAC, without setting `scanCreatesDevice = 0` itself, would otherwise create a `devMac = ''` device, and every other blank-MAC row from every other plugin would then silently write onto it. The filter doesn't replace `scanCreatesDevice = 0` as the correct thing for a plugin to set on such rows; it keeps a MAC-less row inert when some other plugin forgets to. Check any new creation-adjacent query against blank `scanMac` too.
 6. **`app.sql` is not dead code.** `install/production-filesystem/entrypoint.d/25-first-run-db.sh` pipes it into `sqlite3` to bootstrap a brand-new database on first install; `scripts/db_cleanup/regenerate-database.sh` uses it too. `CurrentScan`, `Parameters`, and `Settings` are safe from drift: each has a dedicated `ensure_X()` function (`server/db/db_upgrade.py`) that drops and recreates the table on every startup, superseding whatever `app.sql` bootstrapped. `Plugins_Language_Strings` gets the same treatment inside the shared `ensure_plugins_tables()`. `AppEvents` gets its own drop/recreate via `AppEvent_obj.__init__()` (`server/workflows/app_events.py`), independent of `db_upgrade.py`. `Devices` has no drop/recreate, but `server/database.py` has 18 explicit `ensure_column()` calls that backfill any column missing from an older `app.sql` snapshot on every startup. `Events`, `Sessions`, and `Notifications` get the same backfill via `ensure_table_columns()` (`server/db/db_upgrade.py`), driven by one Python column-list constant per table (`server/db/schema_columns.py`) that's diffed against `app.sql` in CI (`test/db/test_schema_drift_guard.py`). `AppEvents`/`Notifications` each also have a second schema-definition surface — their own inline `CREATE TABLE IF NOT EXISTS` in `server/workflows/app_events.py`/`server/models/notification_instance.py` — kept in sync by the same drift-check test. Check any new query here with `EXPLAIN QUERY PLAN` at a realistic row count rather than assuming it's fine because it resembles an existing one — a correlated subquery re-evaluated per row (an accidental self-join) is the pattern most likely to look reasonable while actually being quadratic at this scale.
 7. **`LatestEventsPerMAC` is intentionally `CurrentScan`-gated - correct for its one existing caller, a trap for a new one.** It `INNER JOIN`s `CurrentScan`, which is exactly right for the mainline "New Connections" query (every MAC it looks up is already known to be in `CurrentScan` this cycle, via its own `present_agg`). It is not a general-purpose "last event for any MAC" lookup. A caller that needs the last event for a MAC *not* in `CurrentScan` this cycle (e.g. a NIC-covered parent with no direct scan row of its own) gets no row at all from this view, regardless of that MAC's real event history - silently, not an error. `insert_events()`'s NIC-derived reconnect query reads `Events` directly instead (a correlated `ORDER BY eveDateTime DESC LIMIT 1`, covered by `idx_eve_mac_datetime_desc`), sidestepping the view entirely rather than trying to make it handle both shapes.
+8. **A correlated helper's own internal subquery alias can be shadowed by an identically-named alias at the call site, silently collapsing its `EXISTS` into a table-wide tautology.** SQL scoping makes the innermost alias declaration win. `nic_derived_presence_condition()` used to alias its own inner scan as `nic_parent`; `insert_events()`'s NIC-derived reconnect query aliases its own row the same way, so `nic_derived_presence_condition("nic_parent.devMac")` silently checked "does any device in `Devices` satisfy this," not "does this one" - any NIC-covered parent anywhere in the table made every other absent parent look NIC-derived-present too. Fixed by renaming the helper's internal alias to `nic_presence_parent` and rejecting it as a `mac_column` argument, matching `current_scan_presence_condition()`'s existing `presence_scan` guard. Only caught by a test with two sibling devices in one DB where one should match and the other shouldn't - every single-device test passed regardless, because "any row" and "this row" are the same row when there's only one.
 
 ## When to read this vs. other docs/skills