Ловим на входе в heroku наши домены поддомены и что надо делаем.

heroku_redirect.rb

# lib/heroku_redirect.rb
class HerokuRedirect
  def initialize(app)
    @app = app
  end

  def call(env)
    request = Rack::Request.new(env)
    unless request.host.starts_with?("www.") || request.host.starts_with?("start.") || request.host.starts_with?("crm.") || request.host.starts_with?("api.") || request.host.starts_with?("erp.") || request.host.include?("mydomain.com") || request.host.include?("staging.")
      # puts 'redirecting to www'
      [301, {"Location" => request.url.sub("//", "//www.")}, self]
    else
      # puts "is erp: #{request.host.starts_with?('erp.')}"
      # puts "is www: #{request.host.starts_with?('www.')}"
      @app.call(env)
    end
  end

  def each(&block)
  end
end

production.rb

# config/environments/production.rb
# ...
  config.middleware.use HerokuRedirect

routes.rb

  scope module: :api do
    constraints subdomain: /api(.*)/ do
      ...
    end
  end