How to specify canonical link in app on heroku when another domain points to it

With a domain pointing to an app on Heroku, I want to set the canonical link to the main domain, not the Heroku url, when the current url is the Heroku url, and not the domain..

How do I do that and not have the canonical link ther开发者_如何学运维e when someone goes to the site at the main domain url?

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You can use one of

ENV['HTTP_HOST']
ENV['SERVER_NAME']
ENV['DOCUMENT_ROOT']

to check whether the site is your main server and if not, print out the canonical URL.

You can also take a look at the ENV variable by dumping it out for more info:

<%= h ENV.inspect %>