cache(name = {}, options = nil, &block) public

This helper exposes a method for caching fragments of a view rather than an entire action or page. This technique is useful caching pieces like menus, lists of new topics, static HTML fragments, and so on. This method takes a block that contains the content you wish to cache.

The best way to use this is by doing key-based cache expiration on top of a cache store like Memcached that’ll automatically kick out old entries. For more on key-based expiration, see: http://37signals.com/svn/posts/3113-how-key-based-cache-expiration-works

When using this method, you list the cache dependency as the name of the cache, like so:

<% cache project do %>
  <b>All the topics on this project</b>
  <%= render project.topics %>
<% end %>

This approach will assume that when a new topic is added, you’ll touch the project. The cache key generated from this call will be something like:

      ^class   ^id ^updated_at    ^template tree digest

The cache is thus automatically bumped whenever the project updated_at is touched.

If your template cache depends on multiple sources (try to avoid this to keep things simple), you can name all these dependencies as part of an array:

<% cache [ project, current_user ] do %>
  <%= render project.topics %>
<% end %>

This will include both records as part of the cache key and updating either of them will expire the cache.

Template digest

The template digest that’s added to the cache key is computed by taking an md5 of the contents of the entire template file. This ensures that your caches will automatically expire when you change the template file.

Note that the md5 is taken of the entire template file, not just what’s within the cache do/end call. So it’s possible that changing something outside of that call will still expire the cache.

Additionally, the digestor will automatically look through your template file for explicit and implicit dependencies, and include those as part of the digest.

The digestor can be bypassed by passing skip_digest: true as an option to the cache call:

<% cache project, skip_digest: true do %>
  <%= render project.topics %>
<% end %>

Implicit dependencies

Most template dependencies can be derived from calls to render in the template itself. Here are some examples of render calls that Cache Digests knows how to decode:

render partial: "comments/comment", collection: commentable.comments
render "comments/comments"
render 'comments/comments'

render "header" => render("comments/header")

render(@topic)         => render("topics/topic")
render(topics)         => render("topics/topic")
render(message.topics) => render("topics/topic")

It’s not possible to derive all render calls like that, though. Here are a few examples of things that can’t be derived:

render group_of_attachments
render @project.documents.where(published: true).order('created_at')

You will have to rewrite those to the explicit form:

render partial: 'attachments/attachment', collection: group_of_attachments
render partial: 'documents/document', collection: @project.documents.where(published: true).order('created_at')

Explicit dependencies

Some times you’ll have template dependencies that can’t be derived at all. This is typically the case when you have template rendering that happens in helpers. Here’s an example:

<%= render_sortable_todolists @project.todolists %>

You’ll need to use a special comment format to call those out:

<%# Template Dependency: todolists/todolist %>
<%= render_sortable_todolists @project.todolists %>

The pattern used to match these is /# Template Dependency: ([^ ]+)/, so it’s important that you type it out just so. You can only declare one template dependency per line.

External dependencies

If you use a helper method, for example, inside of a cached block and you then update that helper, you’ll have to bump the cache as well. It doesn’t really matter how you do it, but the md5 of the template file must change. One recommendation is to simply be explicit in a comment, like:

<%# Helper Dependency Updated: May 6, 2012 at 6pm %>
<%= some_helper_method(person) %>

Now all you’ll have to do is change that timestamp when the helper method changes.

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