# ADR-014: Schema-lean per-site builds (retire cross-plugin FKs, lift plugin tables)

- Status: PROPOSED
- Date: 2026-07-19
- Deciders: cproudlock
- Relates to: ADR-008 (per-plugin migration ownership), ADR-013 (plugin catalog + lean per-site builds), ADR-001 (asset model)

## Context

ADR-013 delivered lean per-site builds for plugin CODE (backend tree + frontend
bundle carry only chosen plugins). One residual was left, explicitly deferred:
the DATABASE. The core Alembic baseline (68b3947ae14f) creates EVERY table,
including ~30 plugin-owned tables (PLUGIN_TABLE_OWNERS). Each plugin's own
baseline is a stamp-only no-op (the core chain already made its tables, per
ADR-008). So a lean site that omits a plugin still creates that plugin's tables,
empty and unused.

The deferral cited a blocker: "the computers-owned installedapps table FKs
machines.machineid while computers declares no dependency on machines; reversing
the cutover would introduce undeclared hard deps or drop FKs; neither is
decided." Investigation refined this:

The cross-boundary foreign keys into the machines plugin table are ALL held by
DEAD legacy tables/columns that predate the asset model (ADR-001) and the
per-plugin cutover (ADR-008), and are queried nowhere in the codebase:

- `machinerelationships` (child/parentmachineid -> machines) - superseded by
  `assetrelationships`. No model, no query.
- `printerdata` (machineid -> machines) - the pre-cutover printers table,
  superseded by the printers plugin. No model, no query.
- `installedapps` (machineid -> machines) - a standalone machine-app link table;
  the live relationship is `computerinstalledapps` (FK to computers only). The
  standalone table has no model, no query.
- `communications.machineid` (-> machines) - a legacy column on the core
  communications table (which is now assetid-based). Not read anywhere.

No LIVE plugin table hard-FKs another plugin's table. computerinstalledapps FKs
only computers.computerid (intra-plugin). So the blocker is dead cruft, not
live design.

## Decision

Two phases, both leaving existing databases correct.

### Phase 1: retire the dead cross-boundary cruft - ALREADY DONE

Investigation found this is already accomplished by existing migrations:
`7a01_adr001_position_contract` and `7c01_drop_legacy_machine` drop
`machinerelationships`, `printerdata`, `installedapps`, and
`communications.machineid` (with its FK). The current schema (verified on the
dev database) has none of them. So the cross-plugin FK blocker ADR-013 cited no
longer exists in the live schema - only in the baseline's transient
create-then-later-drop. No new migration is needed for Phase 1.

Precedent: ADR-001 dropped a cross-plugin FK the same way
(usbcheckouts.machineid -> machines became a soft sentinel).

### Enabling change (executed now): idempotent create_plugin_tables

`shopdb/plugins/alembic_template.py:create_plugin_tables` now skips any table
that already exists (inspects the bind first) instead of raising. This is the
mechanism Phase 2 needs: a plugin anchor can create its tables on a fresh lean
install AND be a safe no-op on an existing database that already has them from
the pre-cutover core baseline. Correct and inert regardless of Phase 2 (no
current caller creates against a populated schema). Verified against the
plugin-migration suite.

### Phase 2 (load-bearing, dedicated pass): lift plugin tables into plugin baselines

With no cross-plugin FKs remaining, each plugin's tables can be created
independently. One coherent baseline edit:

- Remove the ~30 plugin-owned `create_table` blocks from the core baseline
  (68b3947ae14f), plus the four dead-object blocks (machinerelationships,
  printerdata, installedapps, communications.machineid) it creates only for
  later migrations to drop. Core baseline then creates only core tables.
- Change each of the 14 plugin 0001 anchors from stamp-only `pass` to
  `create_plugin_tables(<name>)` / `drop_plugin_tables(<name>)` (idempotent, per
  above).

A fresh lean install then creates core tables plus only the chosen plugins'
tables. A fresh full install creates the identical table set it does today.

Existing-database safety: an existing database is stamped past the baseline and
past each plugin's old stamp-anchor, so neither re-runs; it keeps its tables.
Editing the baseline's content only changes what a FRESH install creates. This
is the highest-blast-radius edit in the project (the released baseline every
site's DB derives from), so it is staged as its own pass gated on the full
verification matrix: fresh-full (== current schema), fresh-lean (strict subset),
existing-DB (no re-run, unchanged), and the migrations-mysql CI (fresh upgrade
from empty + per-plugin install + second-run no-op).

## Consequences

### Positive

- Removes every cross-plugin foreign key; plugin schemas become independent, as
  ADR-013 requires.
- Deletes dead legacy tables/columns every database has carried since the
  cutover (real cleanup, not just lean).
- After Phase 2, a lean site's database contains only core + chosen-plugin
  tables.

### Negative / risk

- Phase 2 edits the released baseline's content. It is safe because existing
  databases never re-run a stamped revision, but it demands the full fresh +
  existing + CI verification and is therefore staged separately.
- Dropping tables is destructive; the migration downgrade recreates them empty
  (structure only) - acceptable because they hold no live data.

## Implementation

- Phase 1: core migration `7d27_retire_legacy_machine_fk_tables` + drop the dead
  `communications.machineid` column from the model. Verified: fresh upgrade,
  idempotent re-run, and a scratch database that had the tables drops them.
- Phase 2: baseline edit + 14 plugin anchor rewrites + idempotent-create guards,
  gated by the migrations-mysql CI, in a dedicated pass.