Zero downtime migration

README.adoc

@@ -91,6 +91,74 @@ end
 migrate_up!
 ----
 
+== Zero Downtime Migration
+
+=== ActiveRecord
+
+Create new columns in database.
+
+[source,ruby]
+----
+class Migration1 < ActiveRecord::Migration
+  def up
+    add_column :users, :encrypted_new_ssn
+    add_column :users, :encrypted_new_ssn_iv
+  end
+end
+----
+
+Add `attr_encrypted` for new columns, `include Transcryptor::ActiveRecord::ZeroDowntime`, and `transcryptor_migrate :old_attribute_name, :new_attribute_name`.
+
+[source,ruby]
+----
+class User < ActiveRecord::Base
+  include Transcryptor::ActiveRecord::ZeroDowntime
+
+  attr_encrypted :ssn, key: '1qwe1qwe1qwe1qwe1qwe1qwe1qwe1qwe', algorithm: 'aes-256-cbc'
+  attr_encrypted :new_ssn, key: '2asd2asd2asd2asd2asd2asd2asd2asd', algorithm: 'aes-256-gcm'
+
+  transcryptor_migrate :ssn, :new_ssn
+end
+----
+
+Create `rake` task for zero downtime migration. Or any other way you prefer.
+
+[source,ruby]
+----
+namespace :zero_downtime
+  desc 'migrate attr_encrypted for User'
+  task user: :environment do
+    User.find_each { |user| user.save! }
+  end
+end
+----
+
+Remove & rename columns in database after finishing of `rake` task.
+
+[source,ruby]
+----
+class Migration2 < ActiveRecord::Migration
+  def up
+    remove_column :users, :encrypted_ssn
+    remove_column :users, :encrypted_ssn_iv
+
+    rename_column :users, :encrypted_new_ssn, :encrypted_ssn
+    rename_column :users, :encrypted_new_ssn_iv, :encrypted_ssn_iv
+  end
+end
+----
+
+Move `attr_encrypted` configuration to original attribute and remove all migration code.
+
+[source,ruby]
+----
+class User < ActiveRecord::Base
+  attr_encrypted :ssn, key: '2asd2asd2asd2asd2asd2asd2asd2asd', algorithm: 'aes-256-gcm'
+end
+----
+
+Done!
+
 == Default Options
 
 Default options for old and new configuration are absolutelly the same as it is defined in `attr_encrypted` gem.

lib/transcryptor/active_record.rb

@@ -3,3 +3,4 @@ module Transcryptor::ActiveRecord
 
 require 'transcryptor/active_record/adapter'
 require 'transcryptor/active_record/re_encrypt_statement'
+require 'transcryptor/active_record/zero_downtime'

lib/transcryptor/active_record/zero_downtime.rb

@@ -0,0 +1,35 @@
+module Transcryptor::ActiveRecord::ZeroDowntime
+  def self.included(base)
+    base.extend ClassMethods
+
+    base.class_eval do
+      after_initialize :transcryptor_migrate
+
+      @transcryptor_migrate_attributes = []
+      class << self
+        attr_reader :transcryptor_migrate_attributes
+      end
+    end
+  end
+
+  def transcryptor_migrate
+    self.class.transcryptor_migrate_attributes.each do |attribute|
+      send("#{attribute[:new]}=", send(attribute[:old]))
+    end
+  end
+
+  module ClassMethods
+    def transcryptor_migrate(old_attribute, new_attribute)
+      @transcryptor_migrate_attributes << {old: old_attribute, new: new_attribute}
+
+      options = attr_encrypted_options.merge(encrypted_attributes[old_attribute])
+      encrypted_attribute_name = "#{options[:prefix]}#{old_attribute}#{options[:suffix]}"
+
+      define_method("#{old_attribute}=") do |value|
+        send("#{encrypted_attribute_name}=", encrypt(old_attribute, value))
+        instance_variable_set("@#{old_attribute}", value)
+        send("#{new_attribute}=", value)
+      end
+    end
+  end
+end

spec/transcryptor/active_record/zero_downtime_spec.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'securerandom'
+
+ActiveRecord::Base.connection.create_table(:active_record_zero_downtime_specs) do |t|
+  t.string   :encrypted_column_1
+  t.string   :encrypted_column_1_iv
+  t.string   :encrypted_new_column_1
+  t.string   :encrypted_new_column_1_iv
+end
+
+class ActiveRecordZeroDowntimeSpec < ActiveRecord::Base
+  include Transcryptor::ActiveRecord::ZeroDowntime
+
+  attr_encrypted :column_1, key: '1qwe1qwe1qwe1qwe1qwe1qwe1qwe1qwe'
+  attr_encrypted :new_column_1, key: '2asd2asd2asd2asd2asd2asd2asd2asd'
+
+  transcryptor_migrate :column_1, :new_column_1
+end
+
+describe Transcryptor::ActiveRecord::ZeroDowntime do
+  let(:instance) { ActiveRecordZeroDowntimeSpec.create(column_1: 'test') }
+
+  it 'assigns encrypted data for both attributes' do
+    expect(instance.new_column_1).to eq('test')
+  end
+
+  it 'changes new attribute on change of old' do
+    instance.column_1 = 'another'
+    expect(instance.new_column_1).to eq('another')
+  end
+end