Schema change on RDB could be a dangerous operation.
That’s the same on Rails, so it’s important to run migrations with bulk option.
This article explains how bulk option changes Rails migration.
Prerequisite
- Rails 6.0
- MySQL 8.0
User table schema
This is users table for a sample.
mysql> desc users;
+------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+----------------+
| id | bigint(20) | NO | PRI | NULL | auto_increment |
| name | varchar(255) | YES | | NULL | |
| created_at | datetime(6) | NO | | NULL | |
| updated_at | datetime(6) | NO | | NULL | |
+------------+--------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)
Migration without bulk option
Let’s add three columns named new_column1, new_column2 and new_column3.
$ bundle exec rails g migration addColumnsToUser new_column1:integer new_column2:string new_column3:boolean
invoke active_record
create db/migrate/20200101164105_add_columns_to_user.rb
The migration file is the following:
class AddColumnsToUser < ActiveRecord::Migration[6.0]
def change
add_column :users, :new_column1, :integer
add_column :users, :new_column2, :string
add_column :users, :new_column3, :boolean
end
end
Run db:migrate.
$ bundle exec rails db:migrate
== 20200101164105 AddColumnsToUser: migrating =================================
-- add_column(:users, :new_column1, :integer)
-> 0.2708s
-- add_column(:users, :new_column2, :string)
-> 0.0496s
-- add_column(:users, :new_column3, :boolean)
-> 0.0347s
== 20200101164105 AddColumnsToUser: migrated (0.3771s) ========================
As you can see, add_column, which may be a dangerous operation, is invoked three times. That’s not good.
Migration with bulk option
Let’s create a migration with bulk option.
Modify the migration file like below:
class AddColumnsToUser < ActiveRecord::Migration[6.0]
def change
change_table :users, bulk: true do |t|
t.integer :new_column1
t.string :new_column2
t.boolean :new_column3
end
end
end
bulk: true means the schema changes will be squashed into one.
Run db:migrate.
$ bundle exec rails db:migrate
== 20200101164105 AddColumnsToUser: migrating =================================
-- change_table(:users, {:bulk=>true})
-> 0.1296s
== 20200101164105 AddColumnsToUser: migrated (0.1300s) ========================
Yes! change_table is invoked just one time!
after option is available
Tips: after option is also available in bulk migration.
class AddColumnsToUser < ActiveRecord::Migration[6.0]
def change
change_table :users, bulk: true do |t|
t.integer :new_column1, after: existing_column1
t.string :new_column2, after: existing_column2
end
end
end