Multiple database handling with Rails

Dec 11

Posted by Sandro Paganotti in Ruby on Rails - comments are closed

What happen if your chief ask you to write an application that use more than one database? Wow… sound scaring, how to tell ActiveRecord that some models belong to a database and others to another? You’ll be surprised ‘cause it is quite easy.

Let’s start:

1. new database description:

Open your database.yml file. Under the three standard databases (development, test and production) add the informations needed to connect to you extra db. Use the standard yml sintax like:


extradb_development:
  adapter:     mysql
  host:        localhost
  username:    root
  password:    blabla
  database:    extradb_dev

extradb_test:
  adapter:     mysql
  host:        localhost
  username:    root
  password:    blabla
  database:    extradb_test

extradb_production:
  adapter:     mysql
  host:        localhost
  username:    root
  password:    blabla
  database:    extradb_prod

2. Create models

Create models using the command line tool as usual, just remember to delete the migration associated.


ruby script/generate model cat 
ruby script/generate model dog

3. Associate models with extradb

You need to manually overwrite the estabilish_connection method for the models linked to the external database. Doing this in ‘application.rb’ using a ‘before_filter’ callback ensure that the connection will be always present. The code is quite simple:


class ApplicationController < ActionController::Base  
  require 'yaml'

  before_filter :set_extra_db_connection

  def set_extra_db_connection
   extra_coord = YAML.load(File.open(File.join(RAILS_ROOT,"config/database.yml"),"r"))["extradb_"+ ENV['RAILS_ENV']]
   Cat.establish_connection(extra_coord)
   Dog.establish_connection(extra_coord)
  end

end

4. Done!

Yess! You made it! Now you can use your ‘foreign models’ exactly like the normal ones. You can use validations and also relationship (also between model from different databases!).

Easy isn’t it?

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