See this issue for a long-running discussion about usage with Heroku (#14).

in database.yml you need to specify the adapter to PostGIS, this value is overridden by the scheme of the URL part. the easiest way it to override that part of the URL.

# config/database.yml
production:
  ...
  adapter: postgis
  url: <%= ENV.fetch("DATABASE_URL").gsub(/postgres/, 'postgis') %>

You need to specify url as Rails will automatically merge configuration values of DATABASE_URL on top of the values provided in database.yml if url config is not set.