In certain scenarios, I've found it useful to use our production app's Read Replica as a source for manual testing. It's not something I'm reaching for often but there are definitely cases where our seed data just doesn't cut it.
Since it's a read replica, no writes can be made to it and I don't have to worry (too much) about harming production. You'll want to make sure any external user operations aren't configured to happen automatically (e.g. sending email, cropping avatars, etc).
Destroy
In order to assign a different DATABASE_URL to a review app, you'll need to destroy the postgres attachment on your review app. I knock this out by chaining some commands together:
heroku addons:destroy $(heroku addons:info heroku-postgresql -a your_review_app | grep '===' | awk '{print $2}') -a your_review_app
The sub-command here is getting the name of the add-on (e.g. postgresql-concentric-04442) and passing it to heroku addons:destroy.1
Configure
Now that we've gotten rid of that pesky, old database, we can set the DATABASE_URL on our review app. I also have a banger of a command for this:
heroku config:set DATABASE_URL="$(heroku config:get HEROKU_POSTGRESQL_PURPLE_URL -r production)" -a your_review_app
In order to know what to substitute in for HEROKU_POSTGRESQL_PURPLE_URL in your setup, run:2
heroku pg:info -r production | grep Followers
Use your discretion to determine your read-replica.
Relish
That's it. In my case, my app was rebooted and it read the DATABASE_URL without a problem. If your app tries to do writes, it will most likely throw an exception.