review-migration

Review Migration

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

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.

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

Safe Patterns and Common Mistakes

Operation	Safe Pattern	Common Mistake	Why It Fails
Add column	Nullable first, backfill later, enforce NOT NULL last	add_column :t, :col, :string, null: false, default: "x" on large table	Table rewrite + lock (PG < 11)
Add index (large table)	algorithm: :concurrently (PG) / :inplace (MySQL) + disable_ddl_transaction!	add_index :users, :email without algorithm: :concurrently	Share lock blocks writes
Backfill data	Batch job outside migration transaction, throttle to reduce replication lag	User.update_all(...) inside migration	Transaction lock held for full duration
Rename column	Add new, copy data, migrate callers, drop old	Rename column directly	Breaks running app during deploy
Add NOT NULL	After backfill confirms all rows have values	Enforce NOT NULL before backfill completes	Fails or locks on rows missing values
Add foreign key	After cleaning orphaned records	Add FK without cleaning orphans	Constraint violation at migration time
Remove column	Remove code references first, deploy, then drop column	Drop column while code still reads it	unknown attribute errors at runtime

For every step, state the expected lock or table-rewrite risk explicitly; if negligible, say why.

Deploy code that tolerates both old and new schemas during transitions.

Code Examples

Concurrent index (Rails / PostgreSQL):

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)
  sleep(0.05) # throttle to reduce replication lag
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

Type change rollout (5-step):

1. Add 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 new column after parity checks pass.
5. Stop writing old column, then drop it in a later deploy.

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

Skill	When to chain
code-review	When reviewing PRs that include migrations
implement-background-job	For backfill jobs that run after schema change
security-check	When migrations expose or move sensitive data