db-defaults-generated

name	db-defaults-generated
description	Audit column defaults and generated columns — application-side timestamps that should be DB defaults, non-deterministic or wrong defaults, derived values that should be GENERATED ALWAYS / computed columns instead of drift-prone duplicated data, and identity/sequence defaults. Module M6. Feeds the Design & Integrity score (Tipos category, shared with M4).
allowed-tools	Read, Grep, Glob, Bash

db-defaults-generated (M6)

Defaults and generated columns are correctness placed at the source: a created_at DEFAULT now() is true for every writer, and a GENERATED ALWAYS AS column can never drift from its inputs. Pushing this logic into the app means each writer can get it wrong differently. This module is design-axis (Tipos category). It applies to engines supporting defaults/generated columns.

What it checks

• Timestamp defaults: created_at/updated_at set only by application code (no DEFAULT now()/CURRENT_TIMESTAMP, no ON UPDATE/trigger) — inconsistent across writers, missing on raw SQL inserts.
• Derived value should be generated: a stored column computed from siblings (full_name, total = qty*price, search_vector) kept in sync by app code rather than GENERATED ALWAYS AS ... STORED — drift-prone (ties to M1 denormalization discipline).
• Non-deterministic / wrong default: defaults that bake in a value that should be dynamic, or a default that masks a missing NOT NULL (e.g. status DEFAULT 'active' hiding required intent), or a DEFAULT '' standing in for NULL.
• Identity/sequence hygiene: serial vs GENERATED ... AS IDENTITY; shared/incorrect sequence ownership.
• Boolean/flag defaults missing, forcing three-valued logic where two was intended.

Axis & severity

• Axis: design; magnitude banded, never invented drift rates.
• Derived stored column maintained by app (drift risk): severity 3, warn, fixable: proposed.
• Missing created_at/updated_at DB default: severity 2–3, warn, fixable: auto (additive default).
• DEFAULT ''/sentinel masking NULL semantics: severity 2, warn.
• M6 holds no sev-5 cap; it shapes the Tipos category value.

Tier-0 static check

Parse DDL/snapshot via scripts/parse-schema.mjs: flag *_at timestamp columns with no DEFAULT; detect stored columns whose name implies derivation (full_name, total, *_count, search_vector) that are plain columns rather than GENERATED; list defaults that are empty-string/sentinel; note serial where identity is preferred. Program-source parses stay directional.

Tier-1 verification query

Inspect column defaults and generated status:

-- $DATABASE_URL, read-only
SELECT table_name, column_name, column_default, is_generated, generation_expression
FROM information_schema.columns
WHERE table_schema NOT IN ('pg_catalog','information_schema')
  AND (column_name ~* '(_at$|total|full_name|count|vector)');

Confirm a "derived" column has drifted from its inputs:

SELECT count(*) AS drifted FROM order_items WHERE total <> qty * unit_price;

Findings

Emit per schema/finding.schema.json. Examples:

• M6.invoices.total_app_maintained — total is a plain column synced by app code, not GENERATED ALWAYS AS ... STORED (severity 3, warn, axis design, fixable: proposed).
• M6.users.created_at_no_default — created_at has no DEFAULT now() (severity 2, warn, fixable: auto, axis design).
• M6.users.name_default_empty_string — DEFAULT '' masks NULL semantics (severity 2, warn). Each finding: evidence.observed quotes the column DDL or default verbatim; verification.reproduce is a runnable query above (method: ddl_parse / schema_introspect / query_stat); expected_impact banded + confidence-tagged.

Honesty

• A DB default does not absolve the app of intent — flag only where DB-side enforcement is clearly safer (timestamps, derived values), not as a blanket "move all defaults to the DB".
• Generated columns have engine/version support limits (Postgres STORED only, MySQL VIRTUAL/STORED); scope the recommendation to the detected engine.
• Never claim a derived column has drifted without a Tier-1 mismatch count; static, it is directional.