redirect to canonical host in Rails 4

Sometimes you want to use a single host name for your app and have requests for other host names redirected to the canonical host.

Example:
myfancyapp.herokuapp.com/* should redirect to myfancyapp.com/*`

You could use rack middleware gem like rack-canonical-host or rack-rewrite.

You can also achieve the same result without any additional gem by using contraints in Rails's routes.

in config/routes.rb

if Rails.env.production?
  constraints(:host => /^(?!myfancyapp\.com)/i) do
    match "/(*path)" => redirect {
      |params, req| "http://myfancyapp.com/#{params[:path]}"
      },  via: [:get, :post]
  end
end

If you – like me – prefer keeping your settings in env variables you can use something along these lines

if ENV['CANONICAL_HOST']
    constraints(:host => Regexp.new("^(?!#{Regexp.escape(ENV['CANONICAL_HOST'])})")) do
      match "/(*path)" => redirect { |params, req| "http://#{ENV['CANONICAL_HOST']}/#{params[:path]}" },  via: [:get, :post]
    end
  end