Follow Law of Demeter when using collections?

Question

Within Ruby on Rails (or any other language with a collection...) is it necessary to break Law of Demeter violations up when querying something simple like a count?

class Survey
  has_one :kingdom

  def first_survey?
    # Should this be broken according to Law of Demeter?
    self.kingdom.surveys.count == 1
    # -or-
    self.kingdom.surveys_count == 1
  end
end

class Kingdom
  has_many :surveys

  # Is this the "right thing to do", or overkill?
  def surveys_count
    self.surveys.count
  end
end

Answer 1

Usually when I see Law of Demeter violations the first question I ask isn't "How do I 'avoid a dot'?" The question you should ask is "Does this functionality that violates belong to someone else?" In this case I would probably structure it like so:

class Survey
  belongs_to :kingdom
end

class Kingdom
  has_many :surveys

  def first_survey?(survey)
    surveys.first == survey
  end
end

kingdom = Kingdom.find(kingdom_id)
first_survey = kingdom.surveys.first
last_survey = kingdom.surveys.last # Assuming there is more than one survey.

kingdom.first_survey?(first_survey) #=> true
kingdom.first_survey?(last_survey)  #=> false

By structuring it this way the Survey no longer has to reach into the Kingdom object and query its assocations, avoiding the Law of Demeter violation. This also allows you to change the definition of first_survey? later on. A contrived example would be if Surveys could be "published". The first_survey? for a Kingdom could then be easily changed to support only checking if the passed in Survey is the first Survey with the published attribute set to true.