Concerning

Bite-sized separation of concerns

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

Using comments:

class Todo < ApplicationRecord # Other todo implementation # ... ## Event tracking has_many :events before_create :track_creation private def track_creation # ... end end 

With an inline module:

Noisy syntax.

class Todo < ApplicationRecord # Other todo implementation # ... module EventTracking extend ActiveSupport::Concern included do has_many :events before_create :track_creation 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 increased overhead can be a reasonable tradeoff even if it reduces our at-a-glance perception of how things work.

class Todo < ApplicationRecord # Other todo implementation # ... include TodoEventTracking end 

Introducing Module#concerning

By quieting the mix-in noise, we arrive at a natural, low-ceremony way to separate bite-sized concerns.

class Todo < ApplicationRecord # Other todo implementation # ... concerning :EventTracking do included do has_many :events before_create :track_creation end private def track_creation # ... end end end Todo.ancestors # => [Todo, Todo::EventTracking, ApplicationRecord, 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.

Prepending concerning

concerning supports a prepend: true argument which will prepend the concern instead of using include for it.

activesupport/lib/active_support/core_ext/module/concerning.rb