Default views in Rails 3.0 with custom resolvers

It is common in Rails 3.0 applications that you want to provide default views for a group of controllers. Let’s say you have a bunch of controllers inside the Admin namespace and you would like each action to fallback to a default template. So if you are rendering the index action for Admin::PostsController and “app/views/admin/posts/index.html.*” is not available, it should then render “app/views/admin/defaults/index.html”.

There are several ways to implement this feature at the controller level. It mainly relies on trying to render the original template and then rescue ActionView::MissingTemplate. If this error is rescued, you then render the default one. However, there is a considerable performance overhead in this approach as it needs to pass through the rendering and template lookup stack twice.

Luckily, since Rails 3.0, we have a new abstraction called resolvers that holds the logic to find a template. I explain comprehensively how resolvers work and their API in my book Crafting Rails Applications. So here I would just show the basics to get this functionality working.
First, we need to define a DefaultResolver, it could be done inside the lib directory:

class MyResolver < ::ActionView::FileSystemResolver
  def initialize
    super("app/views")
  end

  def find_templates(name, prefix, partial, details)
    super(name, "admin/defaults", partial, details)
  end
end

Our new resolver simply inherits from ActionView::FileSystemResolver and does two changes: Overrides the initialize method so the view path defaults to "app/views" inside our application and overrides find_templates. The find_templates method receives the template name, a prefix (i.e. the controller path), a boolean marking if the template is a partial or not and a hash of details. In the example above, we simply ignore the prefix given and hardcode it to "admin/defaults".

Now, assuming that all controllers inside the Admin namespace inherit from an Admin::ApplicationController, we can add default views to all of them by adding the following line:

class Admin::ApplicationController < ActionController::Base
  append_view_path MyResolver.new
end

And we are done! The view_paths holds a list of paths and/or resolvers that the controller will look for templates until one is found. If none is found, an ActionView::MissingTemplate is raised. Since we used append_view_paths, our resolver was added after the "app/views" path, used by default in all controllers.

As you may have guessed, resolvers are a powerful abstraction that allows you to retrieve templates from anywhere, including the database, which is the example given in Crafting Rails Applications.

Finally, template inheritance was a feature recently added to Rails master (upcoming Rails 3.1), so you won't need to create your custom resolver as above. There is a good wrap up about this feature in Rails Edge.

  • http://twitter.com/brmichel Bruno Michel

    Thanks for this article. I’m applying this good stuff to one of my projects where I was using symlinks in app/views for that and it was a bit messy. There is a typo on append_view_path (no ‘s’ at the end), but except that, it was very simple to take your code and adapt it :)

  • http://blog.plataformatec.com.br/ josevalim

    Resolvers are a powerful abstraction that allows you to customize template
    lookup. This post is just one example of the several things you can do with
    resolvers. The book goes through another example and shows how you can
    create your own resolver that searches for templates in the database,
    instead of the filesystem. In other words, you should not ignore this post
    nor the book contents, because they explain and show you how resolvers work.
    You just won’t need to create a resolver as above if you need template
    inheritance.

  • http://blog.plataformatec.com.br/ josevalim

    Glad that you liked and were already able to put it in practice! Typo was
    fixed!