Vertical code slices based on ActiveSupport Dependencies

ActiveSupport::Slices allows to lazily autoload “slices” (or extensions) of Ruby files.

E.g. whenever the application requires the file foo/bar/user.rb through ActiveSupport::Dependencies then it will load all files foo/bar/user_slice.rb that are placed anywhere in the autoload_paths.

To hook up ActiveSupport::Slices just include the gem and require 'active_support/slices'.

Given the following files:

engine-1/app/models/user_slice.rb
engine-2/app/models/user.rb
engine-3/app/models/user_slice.rb

If the application requires “user.rb” either through

1. using the (not yet defined) User constant directly or
2. using require_dependency 'user'

… then ActiveSupport::Dependencies will load engine-2/app/models/user.rb first and ActiveSupport::Slices will load both engine-1/app/models/user_slice.rb and engine-2/app/models/user_slice.rb second.

The Rails < 2.3 engines plugin defined the term “engine” as a “full vertical application slice”. When Rails 2.3 included engines it went a few steps back and only implemented most of the core features, making an “engine” rather a “pimped plugin”. Today in Rails 3 we have engines that are way more powerful and flexible than what we had before – in short they’re just “done right” now.

But if you want to use Rails 3 engines to build full application slices there still is a single, crucial feature missing: the ability to mix first-class-citizen code slices from various engines automatically without having one engine know too much about other engines.

For example:

Let’s say we have a bunch of engines that contribute small applications (say, a blog and a ticket tracker) and only share a few things, maybe a User model. So, we’d have engines like: user, blog, tickets. Obviously users have many blog posts and many tickets.

Now, when the blog engine is installed the User.should have_many(:posts). When the tickets engine is installed the User.should have_many(:tickets). But obviously this needs to be defined in the blog and tickets engines, not in the user engine where the User class is defined.

So, the blog and tickets engines somehow need to reopen the User class and add that association:

# somewhere in the blog engine
User.has_many :posts

# somewhere in the tickets engine
User.has_many :tickets

Ideally we want these code bits to be placed in the app/models directories because that’s the place where people look for model code. And we want these bits to be lazy loaded whenever the application loads the User model.

ActiveSupport::Slices provides a simplistic solution to all of these requirements.

Also see this discussion

If your slices won’t get loaded, make sure they are locatable within your loadpath. If e.g. you want to slice minimal template, you must edit your config/application.rb as follows:

config.autoload_paths += %W(#{config.root}/app/views)