Rubocop complains about Metrics/AbcSize

Question

So I'm trying to take the right path here and understand how to solve this cop, this looks like a small piece of code IMO, why is it complaining?

moving the nested if-else doesn't change anything, any suggestions on how to solve this cop?

 class WebPush::Register

  include Interactor

  # rubocop:disable Metrics/AbcSize
  def call
    user = Learner.find_by(id: context.user_id)

    # return if existing
    if user.web_push_subscription
      context.subscription = user.web_push_subscription
    else
      subscription = WebPushSubscription.new(
        endpoint:   context.push_params[:endpoint],
        auth_key:   context.push_params[:keys][:auth],
        p256dh_key: context.push_params[:keys][:p256dh],
        learner:    user
      )

      if subscription.save
        context.subscription = subscription
      else
        context.error = subscription.errors.full_messages
        context.fail!
      end
    end
  end
  # rubocop:enable Metrics/AbcSize

end

Answer 1

First, you need to understand how ABC is calculated. Nested conditionals do not affect ABC. RuboCop's warning output shows you the calculation results:

Assignment Branch Condition size for call is too high. [<5, 28, 4> 28.72/17]

The <5, 28, 4> is your <Assignments, Branches, Conditionals> , as described in this article .

The overall score is calculated like this: sqrt(5^2 + 28^2 + 4^2) = 28.72

The default maximum score for the cop is 17.

I've annotated your code below with the ABC scores for each line. Take note that every time you reference context , you add a B-point. This is because context is not a local variable to call , so the ABC metric assumes it's a method call every time.

def call
  user = Learner.find_by(id: context.user_id) # <1, 3, 0>

  if user.web_push_subscription # <0, 1, 1>
    context.subscription = user.web_push_subscription # <1, 3, 0>
  else # <0, 0, 1>
    # this constructor call is the most expensive part
    subscription = WebPushSubscription.new( # <1, 1, 0>
      endpoint:   context.push_params[:endpoint], # <0, 3, 0>
      auth_key:   context.push_params[:keys][:auth], # <0, 4, 0>
      p256dh_key: context.push_params[:keys][:p256dh], # <0, 4, 0>
      learner:    user # <0, 0, 0>
    )

    if subscription.save # <0, 1, 1>
      context.subscription = subscription # <1, 2, 0>
    else # <0, 0, 1>
      context.error = subscription.errors.full_messages # <1, 4, 0>
      context.fail! # <0, 2, 0>
    end
  end
end

if you set the cop option: CountRepeatedAttributes: false (which I recommend), it will bring your score down to 19.1 .

In lieu of that, you can bring your score down by extracting the creation of WebPushSubscription into its own method like so:

  def call
    user = Learner.find_by(id: context.user_id)

    if user.web_push_subscription
      context.subscription = user.web_push_subscription
    else
      create_subscription(user)
    end
  end

  private

  def create_subscription(user)
    push_params = context.push_params
    subscription = WebPushSubscription.new(
      endpoint:   push_params[:endpoint],
      auth_key:   push_params.dig(:key, :auth),
      p256dh_key: push_params.dig(:key, :p256dh),
      learner:    user
    )

    if subscription.save
      context.subscription = subscription
    else
      context.error = subscription.errors.full_messages
      context.fail!
    end
  end

This will split the score up between the two methods. Note some additional ABC-saving strategies in create_subscription , like assigning push_params to a variable, and using dig for the nested hash accessors. Final score for create_subscription is between 12 and 16 depending on the cop options you use, and call is between 6 and 8.

Generally all that's required to bring down ABC score is to refactor to smaller methods.

Answer 2

ABC stands for Assignment, Branch, Condition . ABC = sqrt(A^2 + B^2 + C^2)

Let's count them.

  def call
    user = Learner.find_by(id: context.user_id)  // assignment, branches

    # return if existing
    if user.web_push_subscription // branch, condition
      context.subscription = user.web_push_subscription  // assignment, branches
    else  // condition
      subscription = WebPushSubscription.new(  // assignment, branch
        endpoint:   context.push_params[:endpoint],  // branches
        auth_key:   context.push_params[:keys][:auth],  // branches
        p256dh_key: context.push_params[:keys][:p256dh],  // branches
        learner:    user
      )

      if subscription.save // branch, condition
        context.subscription = subscription  // assignment
      else  // condition
        context.error = subscription.errors.full_messages  // assignment, branches
        context.fail!  // branch
      end
    end
  end

• A = 5
• B = 17 (at least)
• C = 4

This gives 18.2. By default, 17 is the maximum Rubcop considers satisfactory.

We can handle this with a few simple extract methods.

  private def user
    @user ||= Learner.find_by(id: context.user_id)
  end

  private def create_web_push_subscription_from_context
    subscription = WebPushSubscription.new(
      endpoint:   context.push_params[:endpoint],
      auth_key:   context.push_params[:keys][:auth],
      p256dh_key: context.push_params[:keys][:p256dh],
      learner:    user
    )

    if subscription.save
      context.subscription = subscription
    else
      context.error = subscription.errors.full_messages
      context.fail!
    end
  end

  def call
    if user.web_push_subscription
      context.subscription = user.web_push_subscription
    else
      create_web_push_subscription_from_context
    end
  end

After this refactoring the code is simpler, each method does one thing. That's what the ABC metric encourages.