Rails multi-tenant / multi-database with subdomains using database.yml

application_controller.rb

  before_filter :select_database

  def select_database
    if Rails.env == "production"
      unless request.subdomain.empty?
        begin
          subdomain = request.subdomains.first
          config    = ActiveRecord::Base.configurations[subdomain]
          if config
            ActiveRecord::Base.establish_connection(config)
          else
            ActiveRecord::Base.establish_connection(Rails.env)
            flash[:error] = "Database not found"
          end
        rescue
          render :text => "Invalid subdomain"
        end
      end
    end
  end

database.yml

common: &common
  adapter: postgresql
  username: username
  password: password
  host: localhost
  encode: utf8

# Subdomains
subdomains: ["sub1", "sub2", "sub3"]

sub1:
  <<: *common  
  database: sub1
sub2:
  <<: *common
  database: sub2
sub3:
  <<: *common
  database: sub3

db.rb

  task :migrate_subdomains do
    configs = YAML::load(File.open(File.expand_path("../../../config/database.yml", __FILE__)))
    configs["subdomains"].each do |subdomain|
      begin
        run "cd #{current_path} && #{rake} db:migrate RAILS_ENV=#{subdomain}"
      rescue Exception => e
        Capistrano::CLI.ui.say "*** [err] #{e.message}"
      end
    end
  end

deploy.rb

after "deploy:migrate", "deploy:migrate_subdomains"