Dynamic ActiveRecord Database Connections (without Rails)

If you are familiar with Ruby on Rails, you know you can configure which database to use based on the RAILS_ENV environment variable. This can be one of several values: production, development, test, etc. You can read more about it in the Rails Environment Settings section in the Ruby on Rails Guide. You can bet, somewhere in the code, Rails is doing some sort of magic with ActiveRecord to tell it which database connection to establish. What if we aren't using Rails?!

tl;dr Skip blog and get straight to the code.

Establishing ActiveRecord Databases

Let's cover the basics. How do we establish a basic database connection?

require 'active_record'

ActiveRecord::Base.establish_connection({
  adapter:  'mysql',
  database: 'mydatabase',
  username: 'user',
  password: 'pass',
  host:     'localhost'
})

The code is fairly straightforward. This tells ActiveRecord to open a connection to a local MySQL database named mydatabase and use user and pass as the login credentials. Now, we can do cool things with our database like create tables and populate them with stuff using models:

ActiveRecord::Schema.define do
  create_table :dogs do |t|
    t.integer :id, :null => false
    t.string  :name
    t.date    :dob
  end
end

class Dog < ActiveRecord::Base
end

Dog.create({
  id:   0,
  name: 'Scruffy',
  dob:  '2012-01-01'
})

Choose a Configuration using Environment Variables

Let's recreate the "magic" in Rails by reading in an environment variable to decide which database connection to open. We do this by using ENV:

require 'active_record'

conf = case ENV['DB']
when 'conf1'
  {
    adapter:  'sqlite3',
    database: 'db/mydb1.sqlite3'
  }
when 'conf2'
  {
    adapter:  'sqlite3',
    database: 'db/mydb2.sqlite3'
  }
else
  raise 'export DB=conf[n]'
end

ActiveRecord::Base.establish_connection conf

Before we run this script, we need to set our environment variable:

export DB=conf1 # or conf2

Now, depending on ENV['DB'], the code will open a connection to either mydb1.sqlite3 or mydb2.sqlite3.

Externalize Database Configurations

Never store the database configurations in your code. If you distribute the code, all of the information will be available for people to see! This may or may not be a security issue in your situation, but it's better to play it safe and store database configurations somewhere else. Let's call our database configuration file database.yml (looks familiar):

conf1:
  adapter:  sqlite3
  database: db/mydb1.sqlite3

conf2:
  adapter:  sqlite3
  database: db/mydb2.sqlite3

Let's tell ActiveRecord to establish a connection to the proper database by loading our YAML file:

require 'active_record'

conf = YAML.load_file('database.yml')

ActiveRecord::Base.establish_connection conf[ENV['DB']]

Success! We can set our environment variable the same way as before, but our code will look in a YAML file for the proper database configuration rather than some hashes in code.