Simple Dependency Injection in Ruby

Basically Dependency Injection is a practice used to remove dependencies or tight coupling between different objects.

To learn more about DI, I recommend you read this article by Martin Fowler and also this topic.

I usually use DI through Constructor Injection, this approach allows you to inject the dependencies passing to the object on the constructor method. But isn’t always necessary to inject the dependencies when to instantiate the class, you can avoid it by using Hash.fetch to declare the default dependencies.

For example, imagine that we have a repository class as a dependency:

class Repository
  def initialize; end
  def save
    'repository class'
  end
end

So here’s an example how I use DI to inject this repository:

class CreateService
  def initialize(klasses = {})
    @repository = klasses.fetch(:repository) { Repository.new }
  end
  def call
    repository.save
  end
  private
   attr_reader :repository
end

Hash.fetch makes it simple to instantiate the service with default dependencies:

p CreateService.new.call
# "repository class"

and it also makes simple the dependency replacement, see an example with another repository:

class OtherRepository
  def initialize; end
  def save
    'other_repository class'
  end
end
p CreateService.new({ repository: OtherRepository.new }).call
# "other_repository class"

One last detail is to use the attributes as private, to prevent dependencies from being exposed:

private
  attr_reader :repository