rails-migration

name	Rails Migration
description	Safely write and run a Rails database migration. Use when the user asks to add or change a column, table, or index, or mentions "migration", "schema", or "db:migrate".

Rails migration

A focused playbook for changing the database schema in a LlamaPress Rails app.

Steps

1. Generate the migration: bin/rails g migration <DescriptiveName> (e.g. AddPublishedAtToPosts).
2. Edit the generated file under db/migrate/. Prefer a reversible change method; use up/down only when the change can't be auto-reversed.
3. Run it: bin/rails db:migrate. Confirm db/schema.rb updated as expected.
4. If it fails, bin/rails db:rollback, fix the migration, and re-run.

Rules & gotchas

• NEVER edit a migration that has already run in production — add a new migration instead.
• Adding a NOT NULL column to an existing table needs a default (or a backfill + a follow-up migration) or it will fail on non-empty tables.
• For large tables, adding an index can lock writes — call it out to the user before running.
• After migrating, restart is usually not required (Rails reloads in dev), but if models don't pick up the change, use the hard restart tool.

Verify

• bin/rails runner "puts <Model>.column_names" to confirm the column exists.
• Re-read db/schema.rb to confirm the schema version bumped.