Changing view formats in rails 3.1 (delivering mobile html formats, fallback on normal html)

I'm creating a mobile site next to our normal html site. Using rails 3.1. Mobile site is accessed in subdomain m.site.com.

I have defined mobile format (Mime::Type.register_alias "text/html", :mobile).

In ApplicationController I have "before_filter :mobile_site_before_filter" that recognizes mobile site and sets format according to it.

def mobile_site_before_filter
  if request.subdomains.first == 'm'
    original_format = request.format
    request.format = :mobile
    @mobile_site = true
    request.formats.push(original_format)
  end
end

In layouts I have 'main.html.erb' and 'main.mobile.erb'. So with mobile sites mobile layout is used.

Now it kind of works ok.

In UserController I have index action which chooses index.html.erb or index.mobile.erb automatically. No extra coding in top of mobile-view needed. Success.

But I have lots of other views where the same template could be used to serve inside mobile layouts but with minor changes.

E.g. in MessagesController the same view would be almost fine for mobile

In index.html.erb
Normal user info, common for mobile and html
<%= render(:partial => 'messages') %>

Rendering messages.html.erb, no need for messages.mobile.erb. mobile view can be done with css

<%# By default self.formats = [:html] because this is .html.erb partial, even in mobile site %>
<%= self.formats = [:mobile, :html] if @mobile_site # How to get rid of this? %>
<%= render(:partial => 'vote_form') %>
<!-- rest of page ... -->

Now i want to render vote_form.[mobile|html].erb depending on the site...

Now the index.html.erb partial would be fine to use with mobile if I could just choose between vote_form.html.erb or vote_form.mobile.erb. I can choose to use mobile partial with using "self.formats = [:mobile, :html] if @mobile_site" in the beginning of index.html.erb . But it feels stupid to write this in beginning of all templates.

So the question is:

Where does the self.formats come in views (what sets it originally) and how can I set that it's always [:mobile, :html] inside the mobile site? Why isn't it the same as i set in controller before_filter? Can I set it somehow in the controller already?

(minor bonus questions)

Is there something wrong in this approach? Using mostly the same html-views and in some specific cases using mobile.erb views instead. Why doesn't this work by default in rails?
Is there other ways to choose to render :mobile views if found with fallback to normal html view? Even across the partials so that html.erb-view tries to use _partial.mobile.erb partials.

Solution

You can register new format for the whole application in your mime type initializers:

 Mime::Type.register_alias "text/html", :mobile

Now you can do something like this in your templates to specify format priority(see How do I render a partial of a different format in Rails?):

<% self.formats = [:mobile, :html] %>

In this case if there is mobile template it will be used for rendering with fallback to ordinary html template. Now you should only determine if user is browsing via mobile browser and conditionally execute this code. Or you can just assign formats value in ApplicationController filter so correct template will be chosen automaticaly.

UPDATE:

It seems like by this time there is no "legal" way to solve this problem using Rails. Here is unclosed issue in the rails repository. There you can find patch that can solve your problem, but it uses private Rails API, so it can be unstable.

Also you can try implement your own view resolver that possibly can solve the problem: http://jkfill.com/2011/03/11/implementing-a-rails-3-view-resolver/