A useful Rails 4 .irbrc file (for use with Rails-project-specific .irbrc file loading capability from http://railsbros.de/2010/11/29/a-custom-irbrc-for-rails-3.html ). See earlier revisions for Rails 3 version.

.irbrc

def show_controller_action_callbacks
  _process_action_callbacks.map(&:filter)
end
alias :action_callbacks :show_controller_action_callbacks

def recognize_path(path, environment = { :method => 'GET', :extras => {} })
  (@named_routes ||= Rails.application.routes).recognize_path(path, environment)
end
alias :recognize_route :recognize_path

def path_helper_for(path, environment = { :method => 'GET', :extras => {} })
  @map_from_routes_to_route_names ||= Rails.application.routes.named_routes.routes.invert
  @request_class ||= Rails.application.routes.request_class
  @set    ||= Rails.application.routes.set
  @router ||= ActionDispatch::Journey::Router.new(@set)

  path = ActionDispatch::Journey::Router::Utils.normalize_path(path) unless path =~ %r{://}

  begin
    env = Rack::MockRequest.env_for(path, { :method => environment[:method] })
  rescue URI::InvalidURIError => e
    raise ActionController::RoutingError, e.message
  end

  req = @request_class.new(env)

  @router.recognize(req) do |route, _matches, params|
    return "#{@map_from_routes_to_route_names[route]}_path"
  end
end

if (['test', 'development']).include?(Rails.env)
  # From http://www.particlewave.com/2012/07/14/using-factorygirl-with-the-rails-console/ :
  def irb_enable_factory_girl
    require 'factory_girl_rails'

    puts 'Now you can call FactoryGirl.{build|build_stubbed|create} methods in this console session.'
  end
end

class Object
  def switch_db( env )
    config = Rails.configuration.database_configuration
    fail ArgumentError, "Invalid environment #{env}" unless config[env]
    ActiveRecord::Base.establish_connection( config[env] )
    Logger.new( STDOUT ).info( "Successfully changed to #{env} environment." )
  end
end