Default URI format is forced over Accept header

[gmile]

Rails forces format, specified in default over the one, provided in Accept header.

This is somewhat counterintuitive. Semantic meaning of default is to yield a fallback option if no other option was provided. However, right now default is forced.

Upon digging up the Rails, codebase, it became apparent that Rails takes the format param as the one having priority over Accept header.

I personally don't think it is correct, but I can't quickly establish anything RFC-like to prove which one should take precedence: the inferred (in my case: default) response content type, or the one provided by the Accept header.

I'm going to provide an example app (via a template) to back my point.

[kostyl]

@gmile will it change respond_with behavior?

[gmile]

@kostyl hmm, no, I don't think anything should break. Things to consider:

1. you must make sure you always pass in .json type with your URLs (do you?),
2. I think browser (actually, some ember middleware) should set Accept: application/json automatically, but I'm not certain,
3. tests pass, so I think we're good at least in some way?

[matthewd]

The :format parameter is, by design, a means of overriding the best guess that would be derived from Accept.

So, setting it as a route's default parameter is not something I'd expect to need a lot.

.. what behaviour were you intending to achieve?

[gmile]

@matthewd I 100% agree with you.

I'd expect the following:

1. if Accept: ... header is given – take it on a priority,
2. if :format is given – take it,
3. if :format is not given – go with default format (if specified),
4. if none of 1–3 worked, return 415 error.

Meanwhile, in our app we've removed defaults: { format: :json } to work over this issue.

[matthewd]

Yeah, that's not going to happen: browsers (for example) always send Accept; the above ordering would entirely break :format.

[gmile]

@matthewd hmm indeed, now I fully understand your point. Thank you!

[KES777]

Actually I came here with the question: Should server relay on Accept header or requested format in URL?

Unfortunately I did not find any RFC or specs that define this precedence explicitly.

My personal opinion is:

Client software may support many different formats and will send all of them in Accept header, but user of this software may prefer different format with lower priority. So typing format in URI he prioritize desired format. Without this client software (browser/user agent) always will abandon user's wish.

So priority should be next:

1. Take format from query. This will give a chance to scripts chose right format:
   /script.pl?format=html or /script.pl?format.json
2. If no format in query then determine format from path: /script.pl
3. If no format determined yet then start content negotiation relaying on Accept- headers

May someone provide proof link for this?