🔥 Multiple databases for Rails

ActiveRecord supports multiple databases, but Rails < 6 doesn’t provide a way to manage them. Multiverse changes this.

Plus, it’s easy to upgrade to Rails 6 when you get there.

Works with Rails 4.2+

Add this line to your application’s Gemfile:

gem 'multiverse'

In this example, we’ll have a separate database for our e-commerce catalog that we’ll call catalog.

The first step is to generate the necessary files.

rails generate multiverse:db catalog

This creates a CatalogRecord class for models to inherit from and adds configuration to config/database.yml. It also creates a db/catalog directory for migrations and schema.rb to live.

rails and rake commands run for the original database by default. To run commands for the new database, use the DB environment variable. For instance:

Create the database

DB=catalog rails db:create

Create a migration

DB=catalog rails generate migration add_name_to_products

Run migrations

DB=catalog rails db:migrate

Rollback

DB=catalog rails db:rollback

Also works for models

DB=catalog rails generate model Product

This generates

class Product < CatalogRecord
end

Only necessary in Rails < 5.2

For web servers that fork, be sure to reconnect after forking (just like you do with ActiveRecord::Base)

In config/puma.rb, add inside the on_worker_boot block

CatalogRecord.establish_connection :"catalog_#{Rails.env}"

In config/unicorn.rb, add inside the before_fork block

CatalogRecord.connection.disconnect!

And inside the after_fork block

CatalogRecord.establish_connection :"catalog_#{Rails.env}"

Rails fixtures work automatically.

Note: Referential integrity is not disabled on additional databases when fixtures are loaded, so you may run into issues if you use foreign keys. Also, you may run into errors with fixtures if the additional databases aren’t the same type as the primary.

After running migrations for additional databases, run:

DB=catalog rails db:test:prepare

Database Cleaner supports multiple connections out of the box.

cleaner = DatabaseCleaner[:active_record, {model: CatalogRecord}]
cleaner.strategy = :transaction
cleaner.cleaning do
  # code
end

Read more here

There are a few features that aren’t supported on additional databases.

• Pending migration check
• schema_cache.yml

Also note that ActiveRecord::Migration.maintain_test_schema! doesn’t affect additional databases.

Rails 6 provides a way to manage multiple databases 🎉

To upgrade from Multiverse, nest your database configuration in config/database.yml:

# this should be similar to default, but with migrations_paths
catalog_default: &catalog_default
  adapter: ...
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  migrations_paths: db/catalog_migrate

development:
  primary:
    <<: *default
    database: ...
  catalog:
    <<: *catalog_default
    database: ...

test:
  primary:
    <<: *default
    database: ...
  catalog:
    <<: *catalog_default
    database: ...

production:
  primary:
    <<: *default
    database: ...
  catalog:
    <<: *catalog_default
    database: ...

Then change establish_connection in app/models/catalog_record.rb to:

class CatalogRecord < ActiveRecord::Base
  establish_connection :catalog
end

And move:

• db/catalog/migrate to db/catalog_migrate
• db/catalog/schema.rb to db/catalog_schema.rb (or db/catalog/structure.sql to db/catalog_structure.sql).

Then remove multiverse from your Gemfile. 🎉

Now you can use the updated commands:

rails db:migrate          # run all
rails db:migrate:catalog  # runs catalog only

Generate migrations with:

rails generate migration add_name_to_products --database=catalog

And models with:

rails generate model Product --database=catalog --parent=CatalogRecord

Happy scaling!

View the changelog

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/multiverse.git
cd multiverse
bundle install
bundle exec rake test