How to set config.action_controller.default_url_options = {:host = '#''} on per environment basis

Okay I figured it out the correct way to write it is

Rails.application.routes.default_url_options[:host] = 'localhost:3000'

:)

Inherit your Application's default_url_options from ActionMailer.

You want to keep things as DRY as possible so, ideally, you don't want to hard code your host and port in multiple places for the same environment, unless your ActionMailer actually uses a different host and port than the rest of your Application.

To set the default_url_options for your entire Application, simply add the following line to your config/environment.rb file (changing MyApp to your app's name):

# Set the default host and port to be the same as Action Mailer.
MyApp::Application.default_url_options = MyApp::Application.config.action_mailer.default_url_options

This will fix your problem and automatically set your Application's default_url_options to the same as your config.action_mailer.default_url_options:

$ MyApp::Application.config.action_mailer.default_url_options
#=> {:host=>"lvh.me", :port=>"3000"}

$ MyApp::Application.default_url_options
#=> {:host=>"lvh.me", :port=>"3000"}

You have to restart your server before the changes to this file takes effect.