v# User service object when:
# 1- The action is complex
# 2- The action reaches across multiple models (e.g. an e-commerce purchase using Order, CreditCard and Customer objects)
# 3- The action interacts with an external service (e.g. posting to social networks)
# 4- The action is not a core concern of the underlying model (e.g. sweeping up outdated data after a certain time period).
# 5- There are multiple ways of performing the action (e.g. authenticating with an access token or password). 

class UserCSVDump < Struct.new(:users)
  def csv
    CSV.generate do |csv|
      csv << %w[id username email]
      users.each do |user|
        csv << [user.id, user.username, user.email]
      end
    end
  end
end

# Another example would be a class to manage authentication