Set host name as an environment variable in Heroku review app - heroku

Question

I'm using the Review Apps feature integrated with Github on Heroku. In one of my apps, I set an environment variable called HOST_NAME . For example, if the site is http://www.purplebinder.com, then HOST_NAME would be set to www.purplebinder.com. It's used in a couple of places where we work with cookies and in our transactional emails.
When I open up a new pull request and spin up a review app, HOST_NAME should be something like purplebinder-pr-27.herokuapp.com.
Is there a way to set this value automatically? The Heroku documentation on review apps says an env var can inherit a value from the parent app or be hardcoded in app.json. Neither of those approaches work here, because the value needs to be different each time, and also different from the parent app.
Heroku also says an env var can be set "through a generator", but doesn't go into detail about what that is.
This question might be a duplicate of Setting ROOT_URL for Review Apps, but nobody answered that one. It's also similar to How to get Heroku app name from inside the app, but the answers there involved running a script after the app was created - here I'd like to set this value as part of the initial build.

Answer 1

From https://devcenter.heroku.com/articles/github-integration-review-apps#heroku_app_name-and-heroku_parent_app_name:
To help with scripting, two special config vars are available to
review apps. If you specify HEROKU_APP_NAME or HEROKU_PARENT_APP_NAME
as required or optional config vars in your app.json file, Heroku will
set those config vars to the new application name and the parent
application name respectively. They will then be available for use in
the postdeploy script so that you can do more advanced bootstrapping
and configuration.
Here is an example app.json file that uses
HEROKU_APP_NAME and HEROKU_PARENT_APP_NAME:
{
"name":"Advanced App",
"scripts": {
"postdeploy": "rake db:setup && bin/bootstrap"
},
"env": {
"HEROKU_APP_NAME": {
"required": true
},
"HEROKU_PARENT_APP_NAME": {
"required": true
}
}
}

Answer 2

If you add the heroku-buildpack-cli to your parent app, then it enables you to set environment variables from your post-deploy script. The command should look something like the following:
heroku config:set HOST_NAME=${HEROKU_APP_NAME}.herokuapp.com --app ${HEROKU_APP_NAME}

Answer 3

Here's an approach ignoring app.json for Rails installations:
in the relative config/<environment>.rb. I personally use production.rb and staging just references it.
if ENV.fetch("HEROKU_APP_NAME", "").include?("staging-pr-")
ENV["APPLICATION_HOST"] = ENV["HEROKU_APP_NAME"] + ".herokuapp.com"
ENV["ASSET_HOST"] = "https://" + ENV["APPLICATION_HOST"]
config.action_mailer.default_url_options = { host: ENV.fetch("APPLICATION_HOST") }
end
...
It's a bit misleading as the heroku environment variables will still have the old variables, but it works.

Answer 4

You can also create review environment for you application copying staging.rb or production.rb from config/environments. This would be useful.
After adding HEROKU_APP_NAME and HEROKU_PARENT_APP_NAME to your app.json, you can easily set;
config.action_mailer.default_url_options = { host: "#{ENV['HEROKU_APP_NAME']}.herokuapp.com" }
config.action_mailer.asset_host = "http://#{ENV['HEROKU_APP_NAME']}.herokuapp.com"
config.action_controller.asset_host = "#{ENV['HEROKU_APP_NAME']}.herokuapp.com"
config.action_cable.url = "wss://#{ENV['HEROKU_APP_NAME']}.herokuapp.com/cable"