remove all heroku env variables except for database_url. Call it with `ruby heroku_clear_env.rb my-app`

heroku_clear_env.rb

require 'json'

app = ARGV[0]

envs_to_ignore = ['DATABASE_URL']

keys =
  JSON.parse(`heroku config --app #{app} --json`)
      .reduce('') do |memo, (key, _value)|
        if envs_to_ignore.include? key
          memo
        else
          memo + "#{key} "
        end
      end

`heroku config:unset --app #{app} #{keys}`