OnlineMigrations

Catch unsafe PostgreSQL migrations in development and run them easier in production.

✅ Detects potentially dangerous operations
✅ Prevents them from running by default
✅ Provides instructions and helpers on safer ways to do what you want

Note: You probably don't need this gem for smaller projects, as operations that are unsafe at scale can be perfectly safe on smaller, low-traffic tables.

See comparison to strong_migrations

• Ruby 2.7+
• Rails 6.1+
• PostgreSQL 9.6+

For older Ruby and Rails versions you can use '< 0.11' version of this gem.

Note: Since some migration helpers use database VIEWs to implement their logic, it is recommended to use structure.sql schema format, or otherwise add some gem (like scenic) to be able to dump them into the schema.rb.

Add this line to your application's Gemfile:

gem 'online_migrations'

And then run:

$ bundle install
$ bin/rails generate online_migrations:install
$ bin/rails db:migrate

Note: If you do not have plans on using background data migrations or background schema migrations features, then you can delete the generated migration and regenerate it later, if needed.

If you're already using background data migrations or background schema migrations, your background migrations tables may require additional columns. After every upgrade run:

$ bin/rails generate online_migrations:upgrade
$ bin/rails db:migrate

Writing a safe migration can be daunting. Numerous articles have been written on the topic and a few gems are trying to address the problem. Even for someone who has a pretty good command of PostgreSQL, remembering all the subtleties of explicit locking can be problematic.

Online Migrations was created to catch dangerous operations and provide a guidance and code helpers to run them safely.

An operation is classified as dangerous if it either:

• Blocks reads or writes for more than a few seconds (after a lock is acquired)
• Has a good chance of causing application errors

Consider the following migration:

class AddAdminToUsers < ActiveRecord::Migration[7.2]
  def change
    add_column :users, :admin, :boolean, default: false, null: false
  end
end

If the users table is large, running this migration on a live PostgreSQL < 11 database will likely cause downtime.

A safer approach would be to run something like the following:

class AddAdminToUsers < ActiveRecord::Migration[7.2]
  # Do not wrap the migration in a transaction so that locks are held for a shorter time.
  disable_ddl_transaction!

  def up
    # Lower PostgreSQL's lock timeout to avoid statement queueing.
    execute "SET lock_timeout TO '5s'" # The lock_timeout duration is customizable.

    # Add the column without the default value and the not-null constraint.
    add_column :users, :admin, :boolean

    # Set the column's default value.
    change_column_default :users, :admin, false

    # Backfill the column in batches.
    User.in_batches.update_all(admin: false)

    # Add the not-null constraint. Beforehand, set a short statement timeout so that
    # Postgres does not spend too much time performing the full table scan to verify
    # the column contains no nulls.
    execute "SET statement_timeout TO '5s'"
    change_column_null :users, :admin, false
  end

  def down
    remove_column :users, :admin
  end
end

When you actually run the original migration, you will get an error message:

⚠️  [online_migrations] Dangerous operation detected ⚠️

Adding a column with a non-null default blocks reads and writes while the entire table is rewritten.

A safer approach is to:
1. add the column without a default value
2. change the column default
3. backfill existing rows with the new value
4. add the NOT NULL constraint

add_column_with_default takes care of all this steps:

class AddAdminToUsers < ActiveRecord::Migration[7.2]
  disable_ddl_transaction!

  def change
    add_column_with_default :users, :admin, :boolean, default: false, null: false
  end
end

It suggests how to safely implement a migration, which essentially runs the steps similar to described in the previous example.

Potentially dangerous operations:

• removing a column
• adding a column with a default value
• backfilling data
• changing the type of a column
• renaming a column
• renaming a table
• creating a table with the force option
• adding a check constraint
• setting NOT NULL on an existing column
• executing SQL directly
• adding an index non-concurrently
• removing an index non-concurrently
• replacing an index
• adding a reference
• adding a foreign key
• adding an exclusion constraint
• adding a unique constraint
• adding a json column
• adding a stored generated column
• using primary key with short integer type
• hash indexes
• adding multiple foreign keys
• removing a table with multiple foreign keys
• mismatched reference column types
• adding a single table inheritance column

Config-specific checks:

• changing the default value of a column

You can also add custom checks or disable specific checks.

❌ Bad

Active Record caches database columns at runtime, so if you drop a column, it can cause exceptions until your app reboots.

class RemoveNameFromUsers < ActiveRecord::Migration[7.2]
  def change
    remove_column :users, :name
  end
end

✅ Good

1. Ignore the column:

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

2. Deploy

3. Wrap column removing in a safety_assured block:

   class RemoveNameFromUsers < ActiveRecord::Migration[7.2]
     def change
       safety_assured { remove_column :users, :name }
     end
   end

4. Remove column ignoring from step 1

5. Deploy

❌ Bad

In earlier versions of PostgreSQL adding a column with a non-null default value to an existing table blocks reads and writes while the entire table is rewritten.

class AddAdminToUsers < ActiveRecord::Migration[7.2]
  def change
    add_column :users, :admin, :boolean, default: false
  end
end

In PostgreSQL 11+ this no longer requires a table rewrite and is safe. Volatile expressions, however, such as random(), will still result in table rewrites.

✅ Good

A safer approach is to:

1. add the column without a default value
2. change the column default
3. backfill existing rows with the new value

add_column_with_default helper takes care of all this steps:

class AddAdminToUsers < ActiveRecord::Migration[7.2]
  disable_ddl_transaction!

  def change
    add_column_with_default :users, :admin, :boolean, default: false
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail.

❌ Bad

Active Record wraps each migration in a transaction, and backfilling in the same transaction that alters a table keeps the table locked for the duration of the backfill.

class AddAdminToUsers < ActiveRecord::Migration[7.2]
  def change
    add_column :users, :admin, :boolean
    User.update_all(admin: false)
  end
end

Also, running a single query to update data can cause issues for large tables.

✅ Good

There are three keys to backfilling safely: batching, throttling, and running it outside a transaction. Use a update_column_in_batches helper in a separate migration with disable_ddl_transaction!.

class AddAdminToUsers < ActiveRecord::Migration[7.2]
  def change
    add_column :users, :admin, :boolean
  end
end

class BackfillUsersAdminColumn < ActiveRecord::Migration[7.2]
  disable_ddl_transaction!

  def up
    update_column_in_batches(:users, :admin, false, pause_ms: 10)
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail. Note: You may consider background data migrations or background schema migrations to run data changes on large tables.

❌ Bad

Changing the type of an existing column blocks reads and writes while the entire table is rewritten.

class ChangeFilesSizeType < ActiveRecord::Migration[7.2]
  def change
    change_column :files, :size, :bigint
  end
end

A few changes don't require a table rewrite (and are safe) in PostgreSQL:

✅ Good

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

Note: The following steps can also be used to change the primary key's type (e.g., from integer to bigint).

A safer approach can be accomplished in several steps:

1. Create a new column and keep column's data in sync:

   class InitializeChangeFilesSizeType < ActiveRecord::Migration[7.2]
     def change
       initialize_column_type_change :files, :size, :bigint
     end
   end

   Note: initialize_column_type_change accepts additional options (like :limit, :default etc) which will be passed to add_column when creating a new column, so you can override previous values.

2. Backfill data from the old column to the new column:

   class BackfillChangeFilesSizeType < ActiveRecord::Migration[7.2]
     disable_ddl_transaction!
   
     def up
       # You can use `backfill_column_for_type_change_in_background` if want to
       # backfill using background migrations.
       backfill_column_for_type_change :files, :size
     end
   
     def down
       # no op
     end
   end

3. Make sure your application works with values in both formats (when read from the database, converting during writes works automatically). For most column type changes, this does not need any updates in the app.

4. Deploy

5. Copy indexes, foreign keys, check constraints, NOT NULL constraint, swap new column in place:

   class FinalizeChangeFilesSizeType < ActiveRecord::Migration[7.2]
     disable_ddl_transaction!
   
     def change
       finalize_column_type_change :files, :size
     end
   end

6. Deploy

7. Finally, if everything works as expected, remove copy trigger and old column:

   class CleanupChangeFilesSizeType < ActiveRecord::Migration[7.2]
     def up
       cleanup_column_type_change :files, :size
     end
   
     def down
       initialize_column_type_change :files, :size, :integer
     end
   end

8. Remove changes from step 3, if any

9. Deploy

❌ Bad

Renaming a column that's in use will cause errors in your application.

class RenameUsersNameToFirstName < ActiveRecord::Migration[7.2]
  def change
    rename_column :users, :name, :first_name
  end
end

✅ Good

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

The "classic" approach suggests creating a new column and copy data/indexes/etc to it from the old column. This can be costly for very large tables. There is a trick that helps to avoid such heavy operations.

The technique is built on top of database views, using the following steps:

1. Rename the table to some temporary name
2. Create a VIEW using the old table name with addition of a new column as an alias of the old one
3. Add a workaround for Active Record's schema cache

For the previous example, to rename name column to first_name of the users table, we can run:

BEGIN;
ALTER TABLE users RENAME TO users_column_rename;
CREATE VIEW users AS SELECT *, first_name AS name FROM users_column_rename;
COMMIT;

As database views do not expose the underlying table schema (default values, not null constraints, indexes, etc), further steps are needed to update the application to use the new table name. Active Record heavily relies on this data, for example, to initialize new models.

To work around this limitation, we need to tell Active Record to acquire this information from original table using the new table name.

Online Migrations provides several helpers to implement column renaming:

1. Instruct Rails that you are going to rename a column:

   OnlineMigrations.config.column_renames = {
     "users" => {
       "name" => "first_name"
     }
   }

   Note: You also need to temporarily enable partial writes (is disabled by default in Active Record >= 7) until the process of column rename is fully done.

   # config/application.rb
   # For Active Record >= 7
   config.active_record.partial_inserts = true
   
   # Or for Active Record < 7
   config.active_record.partial_writes = true

2. Deploy

3. Tell the database that you are going to rename a column. This will not actually rename any columns, nor any data/indexes/foreign keys copying will be made, so will be instantaneous. It will use a combination of a VIEW and column aliasing to work with both column names simultaneously

   class InitializeRenameUsersNameToFirstName < ActiveRecord::Migration[7.2]
     def change
       initialize_column_rename :users, :name, :first_name
     end
   end

4. Replace usages of the old column with a new column in the codebase

5. If you enabled Active Record enumerate_columns_in_select_statements setting in your application (is disabled by default in Active Record >= 7), then you need to ignore old column:

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

6. Deploy

7. Remove the column rename config from step 1

8. Remove the column ignore from step 5, if added

9. Remove the VIEW created in step 3 and finally rename the column:

   class FinalizeRenameUsersNameToFirstName < ActiveRecord::Migration[7.2]
     def change
       finalize_column_rename :users, :name, :first_name
     end
   end

10. Deploy

❌ Bad

Renaming a table that's in use will cause errors in your application.

class RenameClientsToUsers < ActiveRecord::Migration[7.2]
  def change
    rename_table :clients, :users
  end
end

✅ Good

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

The "classic" approach suggests creating a new table and copy data/indexes/etc to it from the old table. This can be costly for very large tables. There is a trick that helps to avoid such heavy operations.

The technique is built on top of database views, using the following steps:

1. Rename the database table
2. Create a VIEW using the old table name by pointing to the new table name
3. Add a workaround for Active Record's schema cache

For the previous example, to rename clients table to users, we can run:

BEGIN;
ALTER TABLE clients RENAME TO users;
CREATE VIEW clients AS SELECT * FROM users;
COMMIT;

As database views do not expose the underlying table schema (default values, not null constraints, indexes, etc), further steps are needed to update the application to use the new table name. Active Record heavily relies on this data, for example, to initialize new models.

To work around this limitation, we need to tell Active Record to acquire this information from original table using the new table name.

Online Migrations provides several helpers to implement table renaming:

1. Instruct Rails that you are going to rename a table:

   OnlineMigrations.config.table_renames = {
     "clients" => "users"
   }

2. Deploy

3. Create a VIEW:

   class InitializeRenameClientsToUsers < ActiveRecord::Migration[7.2]
     def change
       initialize_table_rename :clients, :users
     end
   end

4. Replace usages of the old table with a new table in the codebase

5. Remove the table rename config from step 1

6. Deploy

7. Remove the VIEW created in step 3:

   class FinalizeRenameClientsToUsers < ActiveRecord::Migration[7.2]
     def change
       finalize_table_rename :clients, :users
     end
   end

8. Deploy

❌ Bad

The force option can drop an existing table.

class CreateUsers < ActiveRecord::Migration[7.2]
  def change
    create_table :users, force: true do |t|
      # ...
    end
  end
end

✅ Good

Create tables without the force option.

class CreateUsers < ActiveRecord::Migration[7.2]
  def change
    create_table :users do |t|
      # ...
    end
  end
end

If you intend to drop an existing table, run drop_table first.

❌ Bad

Adding a check constraint blocks reads and writes while every row is checked.

class AddCheckConstraint < ActiveRecord::Migration[7.2]
  def change
    add_check_constraint :users, "char_length(name) >= 1", name: "name_check"
  end
end

✅ Good

Add the check constraint without validating existing rows, and then validate them in a separate transaction:

class AddCheckConstraint < ActiveRecord::Migration[7.2]
  disable_ddl_transaction!

  def change
    add_check_constraint :users, "char_length(name) >= 1", name: "name_check", validate: false
    validate_check_constraint :users, name: "name_check"
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail.

❌ Bad

Setting NOT NULL on an existing column blocks reads and writes while every row is checked.

class ChangeUsersNameNull < ActiveRecord::Migration[7.2]
  def change
    change_column_null :users, :name, false
  end
end

✅ Good

Instead, add a check constraint and validate it in a separate transaction:

class ChangeUsersNameNull < ActiveRecord::Migration[7.2]
  disable_ddl_transaction!

  def change
    add_not_null_constraint :users, :name, name: "users_name_null", validate: false
    validate_not_null_constraint :users, :name, name: "users_name_null"
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail.

A NOT NULL check constraint is functionally equivalent to setting NOT NULL on the column (but it won't show up in schema.rb in Rails < 6.1). In PostgreSQL 12+, once the check constraint is validated, you can safely set NOT NULL on the column and drop the check constraint.

class ChangeUsersNameNullDropCheck < ActiveRecord::Migration[7.2]
  def change
    # in PostgreSQL 12+, you can then safely set NOT NULL on the column
    change_column_null :users, :name, false
    remove_check_constraint :users, name: "users_name_null"
  end
end

Online Migrations does not support inspecting what happens inside an execute call, so cannot help you here. Make really sure that what you're doing is safe before proceeding, then wrap it in a safety_assured { ... } block:

class ExecuteSQL < ActiveRecord::Migration[7.2]
  def change
    safety_assured { execute "..." }
  end
end

❌ Bad

Adding an index non-concurrently blocks writes.

class AddIndexOnUsersEmail < ActiveRecord::Migration[7.2]
  def change
    add_index :users, :email, unique: true
  end
end

✅ Good

Add indexes concurrently.

class AddIndexOnUsersEmail < ActiveRecord::Migration[7.2]
  disable_ddl_transaction!

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

Note: 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.

❌ Bad

While actual removing of an index is usually fast, removing it non-concurrently tries to obtain an ACCESS EXCLUSIVE lock on the table, waiting for all existing queries to complete and blocking all the subsequent queries (even SELECTs) on that table until the lock is obtained and index is removed.

class RemoveIndexOnUsersEmail < ActiveRecord::Migration[7.2]
  def change
    remove_index :users, :email
  end
end

✅ Good

Remove indexes concurrently.

class RemoveIndexOnUsersEmail < ActiveRecord::Migration[7.2]
  disable_ddl_transaction!

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

Note: If you forget disable_ddl_transaction!, the migration will fail.

❌ Bad

Removing an old index before replacing it with the new one might result in slow queries while building the new index.

class AddIndexOnCreationToProjects < ActiveRecord::Migration[7.2]
  disable_ddl_transaction!

  def change
    remove_index :projects, :creator_id, algorithm: :concurrently
    add_index :projects, [:creator_id, :created_at], algorithm: :concurrently
  end
end

Note: If removed index is covered by any existing index, then it is safe to remove the index before replacing it with the new one.

✅ Good

A safer approach is to create the new index and then delete the old one.

class AddIndexOnCreationToProjects < ActiveRecord::Migration[7.2]
  disable_ddl_transaction!

  def change
    add_index :projects, [:creator_id, :created_at], algorithm: :concurrently
    remove_index :projects, :creator_id, algorithm: :concurrently
  end
end

❌ Bad

Rails adds an index non-concurrently to references by default, which blocks writes. Additionally, if foreign_key option (without validate: false) is provided, both tables are blocked while it is validated.

class AddUserToProjects < ActiveRecord::Migration[7.2]
  def change
    add_reference :projects, :user, foreign_key: true
  end
end

✅ Good

Make sure the index is added concurrently and the foreign key is added in a separate migration. Or you can use add_reference_concurrently helper. It will create a reference and take care of safely adding index and/or foreign key.

class AddUserToProjects < ActiveRecord::Migration[7.2]
  disable_ddl_transaction!

  def change
    add_reference_concurrently :projects, :user
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail.

❌ Bad

Adding a foreign key blocks writes on both tables.

class AddForeignKeyToProjectsUser < ActiveRecord::Migration[7.2]
  def change
    add_foreign_key :projects, :users
  end
end

or

class AddReferenceToProjectsUser < ActiveRecord::Migration[7.2]
  def change
    add_reference :projects, :user, foreign_key: true
  end
end

✅ Good

Add the foreign key without validating existing rows:

class AddForeignKeyToProjectsUser < ActiveRecord::Migration[7.2]
  def change
    add_foreign_key :projects, :users, validate: false
  end
end

Then validate them in a separate migration:

class ValidateForeignKeyOnProjectsUser < ActiveRecord::Migration[7.2]
  def change
    validate_foreign_key :projects, :users
  end
end

❌ Bad

Adding an exclusion constraint blocks reads and writes while every row is checked.

class AddExclusionContraint < ActiveRecord::Migration[7.2]
  def change
    add_exclusion_constraint :users, "number WITH =", using: :gist
  end
end

✅ Good

Let us know if you have a safe way to do this (exclusion constraints cannot be marked NOT VALID).

❌ Bad

Adding a unique constraint blocks reads and writes while the underlying index is being built.

class AddUniqueConstraint < ActiveRecord::Migration[7.2]
  def change
    add_unique_constraint :sections, :position, deferrable: :deferred
  end
end

✅ Good

A safer approach is to create a unique index first, and then create a unique key using that index.

class AddUniqueConstraintAddIndex < ActiveRecord::Migration[7.2]
  disable_ddl_transaction!

  def change
    add_index :sections, :position, unique: true, name: "index_sections_on_position", algorithm: :concurrently
  end
end

class AddUniqueConstraint < ActiveRecord::Migration[7.2]
  def up
    add_unique_constraint :sections, :position, deferrable: :deferred, using_index: "index_sections_on_position"
  end

  def down
    remove_unique_constraint :sections, :position
  end
end

❌ Bad

There's no equality operator for the json column type, which can cause errors for existing SELECT DISTINCT queries in your application.

class AddSettingsToProjects < ActiveRecord::Migration[7.2]
  def change
    add_column :projects, :settings, :json
  end
end

✅ Good

Use jsonb instead.

class AddSettingsToProjects < ActiveRecord::Migration[7.2]
  def change
    add_column :projects, :settings, :jsonb
  end
end

❌ Bad

Adding a stored generated column causes the entire table to be rewritten. During this time, reads and writes are blocked.

class AddLowerEmailToUsers < ActiveRecord::Migration[7.2]
  def change
    add_column :users, :lower_email, :virtual, type: :string, as: "LOWER(email)", stored: true
  end
end

✅ Good

Add a non-generated column and use callbacks or triggers instead.

❌ Bad

When using short integer types as primary key types, there is a risk of running out of IDs on inserts. The default type in Active Record < 5.1 for primary and foreign keys is INTEGER, which allows a little over of 2 billion records. Active Record 5.1 changed the default type to BIGINT.

class CreateUsers < ActiveRecord::Migration[7.2]
  def change
    create_table :users, id: :integer do |t|
      # ...
    end
  end
end

✅ Good

Use one of bigint, bigserial, uuid instead.

class CreateUsers < ActiveRecord::Migration[7.2]
  def change
    create_table :users, id: :bigint do |t| # bigint is the default for Active Record >= 5.1
      # ...
    end
  end
end

❌ Bad - PostgreSQL < 10

Hash index operations are not WAL-logged, so hash indexes might need to be rebuilt with REINDEX after a database crash if there were unwritten changes. Also, changes to hash indexes are not replicated over streaming or file-based replication after the initial base backup, so they give wrong answers to queries that subsequently use them. For these reasons, hash index use is discouraged.

class AddIndexToUsersOnEmail < ActiveRecord::Migration[7.2]
  def change
    add_index :users, :email, unique: true, using: :hash
  end
end

✅ Good - PostgreSQL < 10

Use B-tree indexes instead.

class AddIndexToUsersOnEmail < ActiveRecord::Migration[7.2]
  def change
    add_index :users, :email, unique: true # B-tree by default
  end
end

❌ Bad

Adding multiple foreign keys in a single migration blocks writes on all involved tables until migration is completed. Avoid adding foreign key more than once per migration file, unless the source and target tables are identical.

class CreateUserProjects < ActiveRecord::Migration[7.2]
  def change
    create_table :user_projects do |t|
      t.belongs_to :user, foreign_key: true
      t.belongs_to :project, foreign_key: true
    end
  end
end

✅ Good

Add additional foreign keys in separate migration files. See adding a foreign key for how to properly add foreign keys.

class CreateUserProjects < ActiveRecord::Migration[7.2]
  def change
    create_table :user_projects do |t|
      t.belongs_to :user, foreign_key: true
      t.belongs_to :project, foreign_key: false
    end
  end
end

class AddForeignKeyFromUserProjectsToProject < ActiveRecord::Migration[7.2]
  def change
    add_foreign_key :user_projects, :projects
  end
end

❌ Bad

Removing a table with multiple foreign keys blocks reads and writes on all involved tables until migration is completed. Remove all the foreign keys first.

Assuming, projects has foreign keys on users.id and repositories.id:

class DropProjects < ActiveRecord::Migration[7.2]
  def change
    drop_table :projects
  end
end

✅ Good

Remove all the foreign keys first:

class RemoveProjectsUserFk < ActiveRecord::Migration[7.2]
  def change
    remove_foreign_key :projects, :users
  end
end

class RemoveProjectsRepositoryFk < ActiveRecord::Migration[7.2]
  def change
    remove_foreign_key :projects, :repositories
  end
end

Then remove the table:

class DropProjects < ActiveRecord::Migration[7.2]
  def change
    drop_table :projects
  end
end

❌ Bad

Reference columns should be of the same type as the referenced primary key. Otherwise, there's a risk of bugs caused by IDs representable by one type but not the other.

Assuming, there is a users table with bigint primary key type:

class AddUserIdToProjects < ActiveRecord::Migration[7.2]
  def change
    add_column :projects, :user_id, :integer
  end
end

✅ Good

Add a reference column of the same type as a referenced primary key.

Assuming, there is a users table with bigint primary key type:

class AddUserIdToProjects < ActiveRecord::Migration[7.2]
  def change
    add_column :projects, :user_id, :bigint
  end
end

❌ Bad

Adding a single table inheritance column might cause errors in old instances of your application.

class AddTypeToUsers < ActiveRecord::Migration[7.2]
  def change
    add_column :users, :string, :type, default: "Member"
  end
end

After the migration was ran and the column was added, but before the code is fully deployed to all instances, an old instance may be restarted (due to an error etc). And when it will fetch 'User' records from the database, 'User' will look for a 'Member' subclass (from the 'type' column) and fail to locate it unless it is already defined.

✅ Good

A safer approach is to:

1. ignore the column:

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

2. deploy

3. remove the column ignoring from step 1 and apply initial code changes

4. deploy

❌ Bad

Active Record < 7 enables partial writes by default, which can cause incorrect values to be inserted when changing the default value of a column.

class ChangeSomeColumnDefault < ActiveRecord::Migration[7.2]
  def change
    change_column_default :users, :some_column, from: "old", to: "new"
  end
end

User.create!(some_column: "old") # can insert "new"

✅ Good

Disable partial writes in config/application.rb. For Active Record < 7, use:

config.active_record.partial_writes = false

For Active Record 7, use:

config.active_record.partial_inserts = false

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

class MySafeMigration < ActiveRecord::Migration[7.2]
  def change
    safety_assured { remove_column :users, :some_column }
  end
end

Certain methods like execute and change_table cannot be inspected and are prevented from running by default. Make sure what you're doing is really safe and use this pattern.

Read configuring.md.

Read background_data_migrations.md on how to perform data migrations on large tables.

Read background_schema_migrations.md on how to perform background schema migrations on large tables.

Thanks to strong_migrations gem, GitLab and maintenance_tasks gem for the original ideas.

Bug reports and pull requests are welcome on GitHub at https://github.com/fatkodima/online_migrations.

After checking out the repo, run bundle install to install dependencies. Run createdb online_migrations_test to create a test database. Then, run bundle exec rake test to run the tests. This project uses multiple Gemfiles to test against multiple versions of Active Record; you can run the tests against the specific version with BUNDLE_GEMFILE=gemfiles/activerecord_61.gemfile bundle exec rake test.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Alternatives:

• https://github.com/ankane/strong_migrations
• https://github.com/LendingHome/zero_downtime_migrations
• https://github.com/braintree/pg_ha_migrations
• https://github.com/doctolib/safe-pg-migrations

Interesting reads:

• Explicit Locking
• When Postgres blocks: 7 tips for dealing with locks
• PostgreSQL rocks, except when it blocks: Understanding locks
• PostgreSQL at Scale: Database Schema Changes Without Downtime
• Adding a NOT NULL CONSTRAINT on PG Faster with Minimal Locking
• Adding columns with default values to really large tables in Postgres + Rails
• Safe Operations For High Volume PostgreSQL
• Stop worrying about PostgreSQL locks in your Rails migrations
• Avoiding integer overflows with zero downtime

• support MySQL
• support other ORMs

Background migrations:

• extract as a separate gem
• add UI
• support batching over non-integer and multiple columns

This gem was heavily inspired by the strong_migrations and GitLab's approaches to database migrations. This gem is a superset of strong_migrations, feature-wise, and has the same APIs.

The main differences are:

1. strong_migrations provides you text guidance on how to run migrations safer and you should implement them yourself. This new gem has actual code helpers (and suggests them when fails on unsafe migrations) you can use to do what you want. See example for an example.

   It has migrations helpers for:

   • renaming tables/columns
   • changing columns types (including changing primary/foreign keys from integer to bigint)
   • adding columns with default values
   • backfilling data
   • adding different types of constraints
   • and others

2. This gem has a powerful internal framework for running data migrations and schema migrations on very large tables in background.

   For example, you can use background migrations to migrate data that’s stored in a single JSON column to a separate table instead; backfill values from one column to another (as one of the steps when changing column type); or backfill some column’s value from an API.

3. Yet, it has more checks for unsafe changes (see checks).

4. Currently, this gem supports only PostgreSQL, while strong_migrations also checks MySQL and MariaDB migrations.

5. This gem is more flexible in terms of configuration - see config file for additional configuration options.

The gem is available as open source under the terms of the MIT License.