DB Migrate Skill

You make schema changes that survive a deploy without downtime. The check:migrations lint (scripts/check-migrations-safety.ts) is the deterministic gate; you are the judgment that decides whether a flagged change is actually safe and writes the annotation that satisfies it.

The window (why this matters)

A deploy runs the migration, then rolls out the new app image via blue/green. The two are not atomic and cannot be — during cutover the old task set keeps serving against the already-migrated schema. So:

Every migration must be backward-compatible with the app version that is already deployed.

If a migration drops a column the old code still reads, renames one, or adds a NOT NULL the old inserts don't populate, the old code throws until traffic fully shifts — the downtime we're guarding against. You can't fix this by reordering the pipeline; the only fix is discipline.

Expand / contract

Split every breaking change across two deploys:

1. Expand (this PR): additive, backward-compatible schema + code that tolerates both the old and new shape.
2. Contract (a later PR, after expand is fully deployed): remove the old thing, now that nothing reads it.

Never put expand and contract in the same PR. If this PR both removes the code that used a column and drops the column, the old code is still live during cutover — split it.

Per-operation playbook

You want to	Do (deploy 1 = expand)	Do (deploy 2 = contract)
Add a required column	ADD COLUMN nullable or DEFAULT; code writes it	backfill, then SET NOT NULL
Rename a column/table	add the new name; code dual-writes / reads new-then-old	drop the old name
Drop a column/table	stop all reads/writes in code; ship it	DROP (annotate)
Change a column type	add a new column of the new type; dual-write	backfill, swap reads, drop old
Add FK / CHECK	ADD CONSTRAINT ... NOT VALID	VALIDATE CONSTRAINT separately
Index an existing table	COMMIT; breakpoint → SET lock_timeout = 0 → CREATE INDEX CONCURRENTLY IF NOT EXISTS (see packages/db/scripts/migrate.ts)	—
Drop an index	COMMIT; breakpoint → DROP INDEX CONCURRENTLY — plain DROP INDEX takes ACCESS EXCLUSIVE on the table	—
Backfill data	batched + idempotent UPDATE (keyset/WHERE, bounded)	—

A CREATE INDEX, ADD COLUMN, or ADD CONSTRAINT against a table created in the same migration is always safe (no rows, no live traffic) — the lint already suppresses those.

Tracking the contract (don't let it rot)

The contract half is deferred to a later deploy — and that is exactly when it gets forgotten, leaving dead columns, orphaned tables, and NOT NULLs that never land. Every deferred contract must become a durable, greppable TODO.

When an expand defers a drop, leave a contract-pending marker on the legacy column/table in packages/db/schema.ts — that is the file you will be editing when you finally do the drop, so the reminder lives where the work happens:

// contract-pending(after #5035 is fully deployed): drop once permission-check.ts stops reading it
workspaceId: text('workspace_id'),

Format: contract-pending(<precondition>): <what to drop> — <why it's safe once the precondition holds>. The precondition names the PR/release that removes the last reader and must be fully deployed before the contract ships.

• The TODO list is a grep — always accurate, never drifts: grep -rn "contract-pending" packages/db apps/sim. Run it when starting migration work to see what is owed.
• For anything with a real owner or schedule, also open a tracking issue and put its number in the marker.
• Close the loop in the contract PR: the contract migration's -- migration-safe: annotation references the expand, and you delete the contract-pending marker in the same PR:

  -- migration-safe: contract of #5035 — workspace_id readers removed there, deployed 2026-06-10
  ALTER TABLE "permission_group" DROP COLUMN "workspace_id";
  

• An expand merged without a marker for the drop it defers, or a contract merged without removing its marker, is a bug — flag it in review.

The judgment the lint can't do

The lint flags risky shapes; it cannot know whether a given drop is safe right now. For each flagged statement, do the work it can't:

1. Is the dependency gone? Grep the app for the table/column: search apps/sim and packages for the column name, the Drizzle field (camelCase), and the table object. If any live read/write remains, it is not safe — fix the code first.
2. Did the expand already ship? The removal of that read/write must be in a deploy that is already out, not this same PR. If it's in this PR, split: land the code change now, do the destructive migration in a follow-up after it deploys.
3. Backfills: confirm the UPDATE/DELETE is batched (bounded WHERE/keyset, not a single whole-table statement), idempotent (safe to replay — a failed migration re-runs unjournaled files from the top), and safe under concurrent writes from the still-live old app.

Workflow

1. Edit packages/db/schema.ts, then cd packages/db && bunx drizzle-kit generate to produce the SQL. If this is an expand that defers a drop, leave a contract-pending marker on the legacy column (see "Tracking the contract"). If this is the contract, delete the marker it resolves.
2. Hand-edit the generated SQL where the playbook requires it: CONCURRENTLY + COMMIT; breakpoint for indexes on existing tables, NOT VALID for constraints, batching for backfills.
3. Run bun run check:migrations (base defaults to origin/staging).
   • Hard errors (add-not-null-no-default, rename, index-not-concurrent, constraint-not-valid, …): rewrite into expand/contract. Do not try to annotate them away — the lint won't accept it.
   • Annotate tier (drop-table, drop-column, drop-default, set-not-null, alter-type, drop-index): only after you've confirmed steps 1–3 above, add a comment on the line directly above the statement:

     -- migration-safe: `secret` read removed in v0.6.1 (#1234), shipped two deploys ago
     ALTER TABLE "webhook" DROP COLUMN "secret";
     

     The reason must be specific and name the PR/version that removed the dependency. An empty reason fails the lint.
   • Warnings (data-backfill): non-blocking, but confirm the batching/idempotency before merging.
4. Verify locally: cd packages/db && bun run db:migrate against a dev DB.

Hard rule

Never annotate a destructive statement just to make the lint pass. The annotation is a claim that you verified the old code no longer depends on it. If you can't make that claim truthfully, the change belongs in a later deploy — tell the user to split it.