gistfile1.rb

# Set the number of connections AR pool
#
# place in an initializer such as: config/connection_pool.rb

Rails.application.config.after_initialize do
  ActiveRecord::Base.connection_pool.disconnect!

  ActiveSupport.on_load(:active_record) do
    config = Rails.application.config.database_configuration[Rails.env]
    config['pool'] = ENV['DB_POOL'] || 6
    ActiveRecord::Base.establish_connection(config)
  end
end

place in config/connection_pool.rb

not in config/initializers/whatevs?

@hgmnz any initializer should work, unless you're doing something screwy like this in another initializer and order might matter.