engine_variant.rb

class EngineVariant
  def api_representation
    { engine: engine, locale: locale }
  end
end

subscription.rb

class Subscription
  def api_representation
    {
      name: name,
      schedule: schedule.try(:api_representation),
      queries: queries.to_a,
      engine_variants: engine_variants.map(&:api_representation),
      url_fragments: url_fragments.map(&:api_representation)
    }
  end
end

subscription_schedule.rb

class SubscriptionSchedule
  def api_representation
    {
      interval: interval,
      start_date: start_date
    }
  end
end

url_fragment.rb

class URLFragment
  def api_representation
    {
      type: type,
      value: value,
      name: name
    }
  end
end

The way I've recently been defining nested JSON structures for my APIs. I won't argue against it being a bit of a SRP violation, but having the model responsible for returning a representation of itself as a hash seems fine to me. And in practice, this is the easiest way I've found to keep things DRY and put the knowledge of how to represent a particular nested model in the API in a single place.