Connect to Multiple Databases in a Rake Task with Rails

Sometimes you need access to multiple database within a single rake task, for whatever reason. Here's how you do it.

I've run into a few scenarios in which I need to get data from some other space into my rails app. When the data has been in a different structure, I tend to export it to CSV using some GUI, and then parse the CSV and bring it into my current app.

But, in the case where your data is in a similar structure, it's much simpler to read from the old database, store the data to memory, and then save it to your current database.

We're going to do this within a rake task.

Connecting to Multiple Databases within a Rake Task

Database Config

First thing's first: configure your database appropriately. We need a separate entry in your database.yml file for each database.

For example, your database config may look something like this:

config/database.yml

default: &default
  adapter: mysql2
  encoding: utf8
  pool: 5
  username: root
  password: root
  socket: /tmp/mysql.sock

development:
  <<: *default
  database: my_app_development

test:
  <<: *default
  database: my_app_test

Add your additional entry. It works much better when you append the environment to the end of the name. So, if you're going to run this task in development, call it something like old_db_development, like so:

config/database.yml

# ...

old_db_development:
  <<: *default
  database: my_old_db

Connect to Database

You can connect to a database using the following command:

ActiveRecord::Base.establish_connection(database_name)

where database_name is the name of the key in your database.yml file.

So, your rake task setup looks something like this:

desc 'My import task'
task :import do
  ActiveRecord::Base.establish_connection('old_db_development')
  # logic to store old data to memory

  ActiveRecord::Base.establish_connection('development')
  # logic to import stored data
end

An Example

If that's too abstract, let's look at an example. Let's say you backup your posts every day, and you accidentally deleted a post. Here's how we might restore it.

First, assume the following database config:

config/database.yml

default: &default
  adapter: mysql2
  encoding: utf8
  pool: 5
  username: root
  password: root
  socket: /tmp/mysql.sock

development:
  <<: *default
  database: my_app_development

backup_development:
  <<: *default
  database: my_backup_db

The task might look something like this:

lib/tasks/restore_posts.rake

desc 'Restore missing posts'
task :restore_posts do
  # Store old posts in a `posts` array
  ActiveRecord::Base.establish_connection('backup_development')
  old_posts = []
  Post.all.each { |post| old_posts << post }

  # Recreate missing posts from missing ids
  ActiveRecord::Base.establish_connection('development')
  current_posts = Post.all
  missing_ids = old_posts.collect(&:id) - current_posts.collect(&:id)
  missing_posts = old_posts.select { |p| missing_ids.include?(p.id) }
  missing_posts.each { |post| Post.create!(post) }
end