Bite-sized separation of concerns
(Note! Module#concerning is included in Rails 4.1. You can still use this library, but it will defer to Active Support's implementation if available.)
We often find ourselves with a medium-sized chunk of behavior that we'd like to extract, but only mix in to a single class.
Extracting a plain old Ruby object to encapsulate it and collaborate or delegate to the original object is often a good choice, but when there's no additional state to encapsulate or we're making DSL-style declarations about the parent class, introducing new collaborators can obfuscate rather than simplify.
The typical route is to just dump everything in a monolithic class, perhaps with a comment, as a least-bad alternative. Using modules in separate files means tedious sifting to get a big-picture view.
Dissatisfying ways to separate small concerns
class Todo # Other todo implementation # ... ## Event tracking has_many :events before_create :track_creation after_destroy :track_deletion def self.next_by_event # ... end private def track_creation # ... end end
With an inline module:
class Todo # Other todo implementation # ... module EventTracking extend ActiveSupport::Concern included do has_many :events before_create :track_creation after_destroy :track_deletion end module ClassMethods def next_by_event # ... end end private def track_creation # ... end end include EventTracking end
Mix-in noise exiled to its own file:
Once our chunk of behavior starts pushing the scroll-to-understand it boundary, we give in and move it to a separate file. At this size, the overhead feels in good proportion to the size of our extraction, despite diluting our at-a-glance sense of how things really work.
class Todo # Other todo implementation # ... include TodoEventTracking end
By quieting the mix-in noise, we arrive at a natural, low-ceremony way to separate bite-sized concerns.
class Todo # Other todo implementation # ... concerning :EventTracking do included do has_many :events before_create :track_creation after_destroy :track_deletion end class_methods do def next_by_event # ... end end private def track_creation # ... end end end Todo.ancestors # => Todo, Todo::EventTracking, Object
This small step has some wonderful ripple effects. We can
- grok the behavior of our class in one glance,
- clean up monolithic junk-drawer classes by separating their concerns, and
- stop leaning on protected/private for crude "this is internal stuff" modularity.