Decrease setup time when using parallel_tests with rails, postgres and rspec. Significant perf gains when using the SQL schema format or heavyweight custom prepare steps.

parallel.rake

namespace :parallel do
  task(:prepare_pg, [:count] => "db:test:prepare") do |t, args|
    num_processes = ParallelTests.determine_number_of_processes(args[:count])

    original_test_env_number = ENV['TEST_ENV_NUMBER']
    test_db_name = ActiveRecord::Base.configurations['test']['database']
    Postgresql.drop_connections(test_db_name)

    db = ActiveRecord::Base.connection

    (1..num_processes).each do |test_env_number|
      next if test_env_number == 1 # First process uses the main test database.

      # Determine the DB name for the parallel process.
      ENV['TEST_ENV_NUMBER'] = test_env_number.to_s
      db_config_template = ERB.new(File.read(File.join(Rails.root, 'config', 'database.yml')))
      db_config = YAML.load(db_config_template.result)
      ENV['TEST_ENV_NUMBER'] = original_test_env_number
      parallel_test_db_name = db_config['test']['database']

      # Copy the (already prepared) test database.
      puts "Copying #{test_db_name} to #{parallel_test_db_name}"
      db.drop_database(parallel_test_db_name)
      db.create_database(parallel_test_db_name, :template => test_db_name)
    end
  end
end

a nice 1-off solution -> definetely add it to the wiki, but I think it does not look stable enough for general use / other databases