not sure if it works correctly but we tried to tune the AR connections. In a Rails initialiser, we have:
```ruby
# More info:
#   - https://devcenter.heroku.com/articles/concurrency-and-database-connections#threaded-servers
#
Rails.application.config.after_initialize do
  ActiveRecord::Base.connection_pool.disconnect!

  ActiveSupport.on_load(:active_record) do
    config = ActiveRecord::Base.configurations[Rails.env] || Rails.application.config.database_configuration[Rails.env]

    # This configure the maximum number of connection that AR is allowed to boot.
    # Not the number of connection that AR will boot.
    #
    # We're in a multi-threaded environment and AR is using a connection per thread.
    # We don't really know how many thread will be used by the execution of our program,
    # so we don't really know how many DB connections we'll need.
    # So, here, I configure AR with a, hopefully, large enough number, so we'll not have connections problems.
    #
    # It's a shitty thing to do, because unsafe, but it seems to be the best way to do this with Rails...
    #
    # If you encounter some problems, you can increase the multiplicator.
    #
    config['pool'] = 128

    ActiveRecord::Base.establish_connection(config)
  end
end
```