How to read POST data in rack request

When I run the curl command curl -v -H "Content-type: application/json" -X POST -d '{"name":"abc", "id":"12", "subject":"my subject"}' http://localhost:9292 to send a POST request with data to my Rack application, my code prints out {}. That is coming from puts req.POST() in the code below. Why does it print out {} instead of the POST data? And how do I correctly access the POST data in my Rack ap