Pros and cons of using callbacks for domain logic in Rails

Question

What do you see as the pros and cons of using callbacks for domain logic? (I\'m talking in the context of Rails and/or Ruby projects.)

To start the discussion, I wanted

Answer 1

This question right here ( Ignore the validation failures in rspec ) is an excellent reason why to not put logic in your callbacks: Testability.

Your code can have a tendency to develop many dependencies over time, where you start adding unless Rails.test? into your methods.

I recommend only keeping formatting logic in your before_validation callback, and moving things that touch multiple classes out into a Service object.

So in your case, I would move the normalize_card_number to a before_validation, and then you can validate that the card number is normalized.

But if you needed to go off and create a PaymentProfile somewhere, I would do that in another service workflow object:

class CreatesCustomer
  def create(new_customer_object)
    return new_customer_object unless new_customer_object.valid?
    ActiveRecord::Base.transaction do
      new_customer_object.save!
      PaymentProfile.create!(new_customer_object)
    end
    new_customer_object
  end
end

You could then easily test certain conditions, such as if it is not-valid, if the save doesn't happen, or if the payment gateway throws an exception.