Ruby - Pattern for dynamically including modules

When using Single Table Inheritance or STI a huge amount of code is responsible for determining the table name and creating instances, finders and the likes. Sometimes it can be beneficial to use Modules instead of creating a child- class, especially when considering the idea behind Concerns (the pattern before Rails snatched the name), not to mention the gain when using a different ORM that has no option for STI built-in.

Take an event class that is required to behave differently depending on the event-type that is stored in an instance variable.

class Event
  attr_accessor :type

  def run
    case @type
    when 'MouseEvent'
      puts "This is a mouse event"
    when 'KeyboardEvent'
      puts "This is a keyboard event"
    end
  end
end

The use-case is quite similar to STI, create an abstract class and implement all methods in the child-classes. Now let’s do it with Modules.

module MouseEvent
  def run
    puts "This is a mouse event"
  end
end

module KeyboardEvent
  def run
    puts "This is a keyboard event"
  end
end

class Event
  def initialize(type)
    if Module.const_defined?(type)
      self.singleton_class.send(:include, Module.const_get(type))
    end
  end
end

Event.new("MouseEvent").run
This is a mouse event
=> nil

Event.new("KeyboardEvent").run
This is a keyboard event
=> nil

As shown above, the instance copy of Event is modified by send an include. Dealing with nested Modules does not work yet and guarding the const_get seems a bit much for an internal call that is a controlled environment and should never process user input. The improved version looks like this.

class String
  def constantize
    return self.to_s.split('::').reduce(Module){ |m, c| m.const_get(c) }
  end
end

module MouseEvent
  def run
    puts "This is a mouse event"
  end
end

module KeyboardEvent
  def run
    puts "This is a keyboard event"
  end
end

class Event
  def initialize(type)
    self.singleton_class.send(:include, type.constantize)
  end
end

Event.new("MouseEvent").run
This is a mouse event
=> nil

Event.new("KeyboardEvent").run
This is a keyboard event
=> nil

It is pretty and not much code for the gain of it. A word of warning though, as a collegue responded when he first heard the concept:

There be dragons

Modifying the class copy can lead to a rollercoaster ride of debugging pain.