How to migrate from KeyValue to Container backend in mobility gem

One of the tasks I’m currently working on is a migration from KeyValue backend to Container backend in the project powered by mobility gem. Today we’ll try to perform such migration. Of course - you can adapt code below to migrate between other backends.

Firstly, you need to change your backend in the config file config/initializers/mobility.rb if you want to act globally

Mobility.configure do
  plugins do
    # backend :key_value
    backend :container

    # ........
  end
end

or in model

class Tag < ApplicationRecord
  # translates :title, backend: :key_value
  translates :title, backend: :container
end

Let’s collect information about translated models. There are 2 database tables which are used to store translations on key-value backend - mobility_string_translations and mobility_text_translations. Those tables contain data in the following format:

• id - record id
• locale - locale of the translated string/text
• key - translated key, in code sample above, it is title
• value - comments are unnecessary
• translatable_type and translatable_id - class and id of translated model, in code sample above it is Tag with some id
• created_at and updated_at - timestamps

Let’s check which models actually use translations:

app_dev=# select distinct translatable_type from mobility_string_translations;
 translatable_type
-------------------
 Country
 Tag
 Interest
(3 rows)


app_dev=# select distinct translatable_type from mobility_text_translations;
 translatable_type
-------------------
(0 rows)

As you see - there are only strings, but no text translations, so let’s move on.

Next step - let’s add required columns to our tables

rails g migration MigrateToContainerMobilityBackend

class MigrateToContainerMobilityBackend < ActiveRecord::Migration[7.1]
  def change
    %w[tags countries interests].each do |table|
      change_table table.to_sym, bulk: true do |t|
        t.jsonb :translations, default: {}
      end
    end
  end
end

Don’t forget to remove type from mobility DSL in models

class Tag < ApplicationRecord
  extend Mobility

  # translates :title, type: :string, locale_accessors: true
  translates :title, locale_accessors: true
end

Then we will write some code to copy data. You can use it in the rake task, new migration or console - as you wish.

ActiveRecord::Base.connection.execute("select * from mobility_string_translations").each do |tr|
  tr["translatable_type"].constantize.find(tr["translatable_id"]).update!("#{tr["key"]}_#{tr["locale"]}": tr["value"])
end

ActiveRecord::Base.connection.execute("select * from mobility_text_translations").each do |tr|
  tr["translatable_type"].constantize.find(tr["translatable_id"]).update!("#{tr["key"]}_#{tr["locale"]}": tr["value"])
end

Last step - now you can drop unnecessary tables and columns

rails g migration DropMobilityTablesAndUnusedColumns

class DropMobilityTablesAndUnusedColumns < ActiveRecord::Migration[7.1]
  def change
    drop_table :mobility_string_translations
    drop_table :mobility_text_translations

    # if you haven't removed those columns before
    remove_column :tags, :title
    remove_column :interests, :title
    remove_column :countries, :title
  end
end

Easy, huh? I hope this recipe will be useful for you.