Strong Migrations

Catch unsafe migrations at dev time

:tangerine: Battle-tested at Instacart

Installation

Add this line to your application’s Gemfile:

gem 'strong_migrations'

How It Works

Strong Migrations detects potentially dangerous operations in migrations, prevents them from running by default, and provides instructions on safer ways to do what you want. Here’s an example:

=== Dangerous operation detected #strong_migrations ===

ActiveRecord caches attributes which causes problems
when removing columns. Be sure to ignore the column:

class User < ApplicationRecord
  self.ignored_columns = ["some_column"]
end

Deploy the code, then wrap this step in a safety_assured { ... } block.

class RemoveColumn < ActiveRecord::Migration[5.2]
  def change
    safety_assured { remove_column :users, :some_column }
  end
end

Dangerous Operations

The following operations can cause downtime or errors:

• adding a column with a non-null default value to an existing table
• removing a column
• changing the type of a column
• setting a NOT NULL constraint with a default value
• renaming a column
• renaming a table
• creating a table with the force option
• adding an index non-concurrently (Postgres only)
• adding a json column to an existing table (Postgres only)

Also checks for best practices:

• keeping non-unique indexes to three columns or less

The Zero Downtime Way

Adding a column with a default value

Adding a column with a non-null default causes the entire table to be rewritten.

Instead, add the column without a default value, then change the default.

class AddSomeColumnToUsers < ActiveRecord::Migration[5.2]
  def up
    add_column :users, :some_column, :text
    change_column_default :users, :some_column, "default_value"
  end

  def down
    remove_column :users, :some_column
  end
end

Don’t backfill existing rows in this migration, as it can cause downtime. See the next section for how to do it safely.

With Postgres, this operation is safe as of Postgres 11

Backfilling data

To backfill data, use the Rails console or a separate migration with disable_ddl_transaction!. Avoid backfilling in a transaction, especially one that alters a table. See this great article on why.

class BackfillSomeColumn < ActiveRecord::Migration[5.2]
  disable_ddl_transaction!

  def change
    # Rails 5+
    User.in_batches.update_all some_column: "default_value"

    # Rails < 5
    User.find_in_batches do |records|
      User.where(id: records.map(&:id)).update_all some_column: "default_value"
    end
  end
end

Removing a column

ActiveRecord caches database columns at runtime, so if you drop a column, it can cause exceptions until your app reboots. To prevent this:

1. Tell ActiveRecord to ignore the column from its cache

  # For Rails 5+
  class User < ApplicationRecord
    self.ignored_columns = ["some_column"]
  end

  # For Rails < 5
  class User < ActiveRecord::Base
    def self.columns
      super.reject { |c| c.name == "some_column" }
    end
  end

1. Deploy code
2. Write a migration to remove the column (wrap in safety_assured block)

  class RemoveSomeColumnFromUsers < ActiveRecord::Migration[5.2]
    def change
      safety_assured { remove_column :users, :some_column }
    end
  end

1. Deploy and run migration

Renaming or changing the type of a column

A safer approach is to:

1. Create a new column
2. Write to both columns
3. Backfill data from the old column to the new column
4. Move reads from the old column to the new column
5. Stop writing to the old column
6. Drop the old column

One exception is changing a varchar column to text, which is safe in Postgres 9.1+.

Renaming a table

A safer approach is to:

1. Create a new table
2. Write to both tables
3. Backfill data from the old table to new table
4. Move reads from the old table to the new table
5. Stop writing to the old table
6. Drop the old table

Adding an index (Postgres)

Add indexes concurrently.

class AddSomeIndexToUsers < ActiveRecord::Migration[5.2]
  disable_ddl_transaction!

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

If you forget disable_ddl_transaction!, the migration will fail. Also, note that indexes on new tables (those created in the same migration) don’t require this. Check out gindex to quickly generate index migrations without memorizing the syntax.

Rails 5+ adds an index to references by default. To make sure this happens concurrently, use:

class AddSomeReferenceToUsers < ActiveRecord::Migration[5.2]
  disable_ddl_transaction!

  def change
    add_reference :users, :reference, index: false
    add_index :users, :reference_id, algorithm: :concurrently
  end
end

For polymorphic references, add a compound index on type and id.

Adding a json column (Postgres)

There’s no equality operator for the json column type, which causes issues for SELECT DISTINCT queries.

If you’re on Postgres 9.4+, use jsonb instead.

If you must use json, replace all calls to uniq with a custom scope.

class User < ApplicationRecord
  scope :uniq_on_id, -> { select("DISTINCT ON (users.id) users.*") }
end

Then add the column:

class AddJsonColumnToUsers < ActiveRecord::Migration[5.2]
  def change
    safety_assured { add_column :users, :some_column, :json }
  end
end

Assuring Safety

To mark a step in the migration as safe, despite using method that might otherwise be dangerous, wrap it in a safety_assured block.

class MySafeMigration < ActiveRecord::Migration[5.2]
  def change
    safety_assured { remove_column :users, :some_column }
  end
end

Custom Checks

Add your own custom checks with:

StrongMigrations.add_check do |method, args|
  if method == :add_index && args[0].to_s == "users"
    stop! "No more indexes on the users table"
  end
end

Use the stop! method to stop migrations.

Existing Migrations

To mark migrations as safe that were created before installing this gem, create an initializer with:

StrongMigrations.start_after = 20170101000000

Use the version from your latest migration.

Dangerous Tasks

For safety, dangerous rake tasks are disabled in production - db:drop, db:reset, db:schema:load, and db:structure:load. To get around this, use:

SAFETY_ASSURED=1 rake db:drop

Faster Migrations

Only dump the schema when adding a new migration. If you use Git, create an initializer with:

ActiveRecord::Base.dump_schema_after_migration = Rails.env.development? &&
  `git status db/migrate/ --porcelain`.present?

Schema Sanity

Columns can flip order in db/schema.rb when you have multiple developers. One way to prevent this is to alphabetize them. Add to the end of your Rakefile:

task "db:schema:dump": "strong_migrations:alphabetize_columns"

Custom Messages

To customize specific messages, create an initializer with:

StrongMigrations.error_messages[:add_column_default] = "Your custom instructions"

Check the source code for the list of keys.

Analyze Tables (Postgres)

Analyze tables automatically (to update planner statistics) after an index is added. Create an initializer with:

StrongMigrations.auto_analyze = true

Lock Timeout (Postgres)

It’s a good idea to set a lock timeout for the database user that runs migrations. This way, if migrations can’t acquire a lock in a timely manner, other statements won’t be stuck behind it. Here’s a great explanation of how lock queues work.

ALTER ROLE myuser SET lock_timeout = '10s';

There’s also a gem you can use for this.

Bigint Primary Keys (Postgres & MySQL)

Rails 5.1+ uses bigint for primary keys to keep you from running out of ids. To get this in earlier versions of Rails, check out rails-bigint-primarykey.

Additional Reading

• Rails Migrations with No Downtime
• Safe Operations For High Volume PostgreSQL

Credits

Thanks to Bob Remeika and David Waller for the original code.

Contributing

Everyone is encouraged to help improve this project. Here are a few ways you can help:

• Report bugs
• Fix bugs and submit pull requests
• Write, clarify, or fix documentation
• Suggest or add new features

To get started with development and testing:

git clone https://github.com/ankane/strong_migrations.git
cd strong_migrations
bundle install
bundle exec rake test