I changed the custom domain on my Heroku app to a new one. Now I will create a new Heroku app which only purpose will be to redirect to the first app.
I read in Google
In your controller action:
redirect_to "http://new.com#{request.request_uri}", :status => 301
However, Heroku has what may be a slightly better option for you documented in their dev center:
class ApplicationController
before_filter :ensure_domain
APP_DOMAIN = 'myapp.mydomain.com'
def ensure_domain
if request.env['HTTP_HOST'] != APP_DOMAIN
# HTTP 301 is a "permanent" redirect
redirect_to "http://#{APP_DOMAIN}#{request.request_uri}", :status => 301
end
end
end