Review Migration

Review Migration

Use this skill when schema changes must be safe in real environments.

Quick Reference

HARD-GATE

DO NOT combine schema change and data backfill in one migration.
DO NOT add NOT NULL on a column that hasn't been fully backfilled.
DO NOT drop columns before all code references are removed.

Core Process

1. Identify the database and table-size risk.
2. Separate schema changes from data backfills.
3. Check lock behavior for indexes, constraints, defaults, and rewrites.
4. Plan deployment order between app code and migration code.
5. Plan rollback or forward-fix strategy.

Safe Patterns

• Deploy code that tolerates both old and new schemas during transitions.
• Add indexes concurrently when supported.
• Backfill in batches outside a long transaction when volume is high.
• Use multi-step rollouts for renames, type changes, and unique constraints.
• For every step, state the expected lock or table-rewrite risk explicitly; if negligible, say why.

If the project uses strong_migrations, follow it. If it does not, apply the same safety rules manually.

Type change rollout pattern:

1. Add the new typed column as nullable.
2. Dual-write old and new columns from application code.
3. Backfill in batches outside the migration transaction.
4. Read from the new column after parity checks pass.
5. Stop writing the old column, then drop it in a later deploy.

Code Examples

Concurrent index (Rails / PostgreSQL):

# Migration file
class AddIndexOnUsersEmail < ActiveRecord::Migration[7.1]
  disable_ddl_transaction!

  def change
    add_index :users, :email, algorithm: :concurrently
  end
end

disable_ddl_transaction! is required — concurrent index creation cannot run inside a transaction.

Nullable-first column with deferred NOT NULL (Rails):

# Step 1 — Deploy: add nullable column
class AddConfirmedAtToUsers < ActiveRecord::Migration[7.1]
  def change
    add_column :users, :confirmed_at, :datetime
  end
end

# Step 2 — Backfill outside migration (background job or script)
User.in_batches(of: 1_000) do |batch|
  batch.update_all(confirmed_at: Time.current)
end

# Step 3 — Deploy: enforce NOT NULL only after all rows are filled
class ChangeConfirmedAtNotNull < ActiveRecord::Migration[7.1]
  def change
    change_column_null :users, :confirmed_at, false
  end
end

Batch backfill snippet (safe for large tables):

# Run this as a Rake task or background job, never inside a migration transaction
BATCH_SIZE = 1_000

User.where(confirmed_at: nil).in_batches(of: BATCH_SIZE) do |batch|
  batch.update_all(confirmed_at: Time.current)
  sleep(0.05) # throttle to reduce replication lag
end

Common Mistakes

Output Style

1. List risks first.
2. For each risk include: Migration step, likely failure mode, explicit lock/table-rewrite risk, safer rollout, rollback or forward-fix note.
3. Ensure backwards compatibility steps are included.
4. Always include explicit phased patterns for column renames, type changes, and unique constraints. If one does not apply, mark it Not applicable and explain why.
5. Language — Must be in English unless explicitly requested otherwise.

Integration

Additional Resources

• PATTERNS.md — Advanced migration patterns for complex schema operations