```ruby
# Default Puma config taken from Heroku, see:
# https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server

workers Integer(ENV['WEB_CONCURRENCY'] || 1)
threads_count = Integer(ENV['MAX_THREADS'] || 5)
threads threads_count, threads_count

preload_app!

rackup      DefaultRackup
port        ENV['PORT']     || 3000
environment ENV['RACK_ENV'] || 'development'

on_worker_boot do
  # Worker specific setup for Rails 4.1+
  # See:
  # https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server#on-worker-boot
  ActiveRecord::Base.establish_connection
end
```

Set Rails environment via RAILS_ENV instead RACK_ENV.
user
```ruby
# Default Puma config taken from Heroku, see:
# https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server

workers Integer(ENV['WEB_CONCURRENCY'] || 1)
threads_count = Integer(ENV['MAX_THREADS'] || 5)
threads threads_count, threads_count

preload_app!

rackup      DefaultRackup
port        ENV['PORT']      || 3000
environment ENV['RAILS_ENV'] || 'development'

on_worker_boot do
  # Worker specific setup for Rails 4.1+
  # See:
  # https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server#on-worker-boot
  ActiveRecord::Base.establish_connection
end
```
assistant