How does rails determine incoming request format?

Question

I'm just wondering how rails knows the format of the request as to correctly enter in the famous:

respond_to do |format|
  format.html
  format.xml
  format.json
end

As an example consider this situation I have faced up. Suppose that via javascript (using jQuery) I make a POST request expliciting dataType: json

$.ajax({
      type: 'POST',
      url: 'example.com',
      data: data,
      dataType: 'json'
    });

When this request reach controller action, standing inside it with ruby debugger, I inspect @request.format and I can see that content-type is application/json. Then the controller respond to json format as expected.

But I'm confused with the format symbol especified in the routes. Suppose that a request is made to example.com/parts.json but in the request the content type is application/html or application/xml. Is the controller responding to json format or html or xml??

Thanks!

Answer 1

The incoming Content-Type only affects the way the request is parsed. It doesn't affect the response format.

Since Rails 5.0, the response format is determined by checking for:

1. A format parameter (e.g. /url?format=xml)
2. The HTTP Accept header (e.g. Accept: application/json)
3. The path extension (e.g. /url.html)

You can see this in the implementation of ActionDispatch::Http::MimeNegotation#formats. Here is an excerpt from Rails v6.1:

if params_readable?
  Array(Mime[parameters[:format]])
elsif use_accept_header && valid_accept_header
  accepts
elsif extension_format = format_from_path_extension
  [extension_format]
elsif xhr?
  [Mime[:js]]
else
  [Mime[:html]]
end