Some time ago I read the book Practical Object-Oriented Design in Ruby by Sandi Metz, while an excellent book there is one thing that bothered me while reading it.

That is the pattern for dependecy injection proposed on the book. It goes something like this:

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  class Car
    def initialise(engine)
      @engine = engine
    end

    def start
      engine.start
    end
  end

  car = Car.new(engine)

In this example the class Car takes an engine dependecy, each time you want to create a car you have to pass an engine to it. This can easily become a cumbersome way of instantiating objects because you need to know a lot about your object just to create them. In this case I need to know that Car takes an engine and specifically which engine depending on the type of car I want to create.

To deal with this the suggested solution is to use an injector. See the definition in wikipedia. The injector is in charge of creating the instances you need and injecting the right dependecies.

Although this is a powerful pattern I believe it mostly an unnecessary complication, specially in dynamic languages like ruby. Let me explain why.

What do we want to achieve with DI?

The main reason for using DI is to be able to swap the dependencies of an object at run time, this is mostly useful for testing. To achieve this there is a dead simple way to do it without having to deal with the complexity of injectors.

The simple way to do it

Instead of passing the dependecy when instantiating an object, we can simply let the object have default dependecies.

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  class Car

    attr_writer :engine

    def start
      engine.start
    end

    def engine
      @engine ||= Engine.new
    end
  end

  car = Car.new

If we just create a car we get a default engine for it, so we don’t need to bother with injectors.

What do we gain:

• External code doesn’t need to know about the internals of our objects
• We have sane default dependecies
• We can easily swap the dependecies at run time

In your test you can simply do this:

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  car = Car.new
  car.engine = FakeEngine

  car.start

So that’s it, DI doesn’t have to be complicated at all in ruby.