Look up a Rails HTTP Status Code

400 - Bad Request

Rails HTTP Status code :bad_request

When to use status code 400

The server cannot or will not process the request due to something that is perceived to be a client error (e.g., malformed request syntax, invalid request message framing, or deceptive request routing).

More about HTTP status codes at

https://en.wikipedia.org/wiki/List_of_HTTP_status_codes
https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

More about rails rendering at

http://guides.rubyonrails.org/layouts_and_rendering.html