How does rails determine incoming request format?

问题

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!

回答1:

From ActionController::MimeResponds: "Rails determines the desired response format from the HTTP Accept header submitted by the client."

来源：https://stackoverflow.com/questions/6253826/how-does-rails-determine-incoming-request-format