How to use multiple databases for one rails 3.1 app in Heroku?

Working off of the previous responses, but incorporating some Rails 3 advantages with the configuration and simplifying the parsing...

# config/application.rbmodule MyApp  class Application < Rails::Application    ... other configs    config.secondary_database_url = ENV['SECONDARY_DB_URL']  endend

We may want to override this in development / test

# config/environments/development.rbmodule MyApp  class Application < Rails::Application    ... other configs    config.secondary_database_url = 'SOME_CONNECTION_STRING'  endend    

Now to setup the class we'll have our models inherit from...

# lib/active_record/secondary.rb module ActiveRecord  class Secondary < ActiveRecord::Base    self.abstract_class = true    # prior to AR 3.2.1    url = URI.parse( MyApp::Application.config.secondary_database_url )    establish_connection(      :adapter  => 'mysql',      :host     => url.host,      :username => url.userinfo.split(':')[0],      :password => url.userinfo.split(':')[1],      :database => url.path[1..-1],      :port     => url.port || 3306    )    # as of AR 3.2.1    establish_connection(MyApp::Application.config.secondary_database_url)  end  class SecondaryMigration < ActiveRecord::Migration    def connection      ActiveRecord::Secondary.connection     end  endend

Heroku will always connect your app to the production DB that they create for you. If you want to make an additional connection you'll need to do this in your code manually, and create a ENV var that the code can use as a connection string.

Anything in the production segment of database.yml is binned by Heroku and replaced.

Regarding Neil's answer, here is a way to do it. Not an out-of-box solution, but might give you an idea.../lib/active_record_extensions.rb

module ActiveRecordExtensions  class Shard < ActiveRecord::Base    #need to switch to the shard database connection from heroku config     primary_database_url = ENV['PRIMARY_DATABASE_URL']    if(!primary_database_url.nil?)      parsed_connection_string = primary_database_url.split("://")      adapter = parsed_connection_string[0]      parsed_connection_string = parsed_connection_string[1].split(":")      username = parsed_connection_string[0]      parsed_connection_string = parsed_connection_string[1].split("@")      password = parsed_connection_string[0]      parsed_connection_string = parsed_connection_string[1].split("/")        host = parsed_connection_string[0]      database = parsed_connection_string[1]      establish_connection(        :adapter  => adapter,        :host     => host,        :username => username,        :password => password,        :database => database,        :port     => 3306,        :pool     => 5,        :timeout  => 5000      )    else      self.establish_connection "shard_#{Rails.env}"    end  end  class ShardMigration < ActiveRecord::Migration    def connection      ActiveRecord::Shard.connection     end  endend

So your model should just extend ActiveRecord::Shard instead of Base