What does “WARN Could not determine content-length of response body.” mean and how to I get rid of it?

Question

Since upgrading to Rails 3.1 I\'m seeing this warning message in my development log:

WARN Could not determine content-length of response body. Set co

Answer 1

Add

config.middleware.use Rack::ContentLength

to your application.rb file, and the warning will disappear even with webrick. This will also set Content-Length properly in production when rendering a json or text response.