Use dotenv with unicorn (put in config/boot.rb, thanks @keithpitt)

puma.rb

class PumaHelper
  def self.refresh_connection_pool(base_klass, config_key)
    base_klass = base_klass.constantize
    config     = Rails.application.config.database_configuration[config_key]

    base_klass.connection_pool.disconnect! rescue ActiveRecord::ConnectionNotEstablished
    base_klass.connection.disconnect!      rescue ActiveRecord::ConnectionNotEstablished
    base_klass.establish_connection config
    true
  rescue
    false
  end
end

threads ENV.fetch('NIBBLER_PUMA_MIN_THREADS').to_i, ENV.fetch('NIBBLER_PUMA_MAX_THREADS').to_i
workers ENV.fetch('NIBBLER_PUMA_WORKERS').to_i
bind ENV.fetch('NIBBLER_PUMA_SOCKET_DIR')
pidfile ENV.fetch('NIBBLER_PUMA_PIDFILE')
state_path ENV.fetch('NIBBLER_PUMA_STATE_PATH')

preload_app!

on_worker_boot do
  if defined?(ActiveRecord::Base)
    ActiveSupport.on_load(:active_record) do
      PumaHelper.refresh_connection_pool 'ActiveRecord::Base', rails_env
    end
  end
end

activate_control_app