Using Rails Migration on different database than standard "production" or "development"

There's a much easier answer. Add this to your migration:

def connection
  ActiveRecord::Base.establish_connection("quiz_#{Rails.env}").connection
end

That's for Rails 3.1. For Rails 2.X or 3.0 it's a class function instead (eg def self.connection)

I got this to work with the following code.

class AddInProgressToRefHighLevelStatuses < ActiveRecord::Migration
  def connection
    @connection = ActiveRecord::Base.establish_connection("sdmstore_#{Rails.env}").connection
  end

  def change
    add_column :ref_high_level_statuses, :is_in_progress, :boolean, :default => true

    @connection = ActiveRecord::Base.establish_connection("#{Rails.env}").connection
  end
end

It was necessary to set the connection back to get it to write the migration to the schema_migrations table so rake would not try to re-run the migration the next time. This assumes that you want the schema_migrations table in the default database configuration to keep track of the migrations checked into version control for the corresponding project.

I was unable to get the down migration to work.

You should define the other databases/environments in /config/environments.

After that you can use the following command to migrate that specific environment.

rake db:migrate RAILS_ENV=customenvironment

I recently struggled with the same problem. The goal was to split off a histories table to a different database since it was already so large and still growing very quickly.

I started trying to resolve it by doing ActiveRecord::Base.establish_connection(:history_database), but could not get any variations of that way to work without the connection being closed. Then finally I discovered the solution below.

In the History model after making this change:

class History < ActiveRecord::Base

  # Directs queries to a database specifically for History
  establish_connection :history_database

  ...
end

I was able to do this in the migration and it worked perfectly:

class CreateHistoriesTableInHistoryDatabase < ActiveRecord::Migration
  def up
    History.connection.create_table :histories do |t|
      ...
    end
  end

  def down
    History.connection.drop_table :histories
  end
end

This will create the table in a different database, yet modify the schema_migrations table in the original database so the migration does not run again.