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Fighting the primitive obsession with Value objects

— Published originally on blog.arkency.com

My previous post on read models intended to address something different, but I decided to focus on read model part and leave the other topic for a different one. There's one thing which I dislike in the implementation. Using primitives to calculate the scores.

Projection

def calculate_scores(test_id, participant_id)
  RailsEventStore::Projection
    .from_stream(stream_name(test_id, participant_id))
    .init(-> { Hash.new { |scores, skill_id| scores[skill_id] = { score: 0, number_of_scores: 0 } })
    .when(
      SurveyExecution::AnswerRegistered,
      ->(state, event) do
        skill_id = event.data.fetch(:skill_id)
        state[skill_id][:score] += event.data.fetch(:score)
        state[skill_id][:number_of_scores] += 1
      end
    )
    .run(Rails.configuration.event_store)
    .reduce({}) do |scores, (skill_id, values)|
      scores[skill_id] = values[:score] / values[:n]
      scores
    end
end

It accumulates the score in scope of a given skill so we can count the average and so on. This example is simplified, as you may suspect, the original is more complex.

We can do better

How can it be done differently? By introducing Value object. Before diving into the code, we should establish the correct definition of it. I like characteristics of Value object which Eric Evans put in his „Domain-Driven Design: Tackling the Complexity in the Heart of Software” book:

  • It measures, quantifies, or describes a thing in the domain.
  • It can be maintained as immutable.
  • It models a conceptual whole by composing related attributes as an integral unit.
  • It is completely replaceable when the measurement or description changes.
  • It can be compared with others using Value equality.
  • It supplies its collaborators with Side-Effect-Free Behavior

Probably the most common example of Value object you'll meet is the Price or MonetaryValue which represents the combo of BigDecimal and a String representing the currency. I'll do something different then.

class AnswerScore
  def initialize(skill_id, score)
    @skill_id = skill_id
    @score = BigDecimal(score.to_s)
  end

  attr_reader :skill_id, :score

  def eql?(other)
    other.instance_of?(AnswerScore) && skill_id.eql?(other.skill_id) && score.eql?(other.score)
  end

  alias == eql?

  def hash
    AnswerScore.hash ^ [skill_id, score].hash
  end
end

What we got here, we are able to compare two different AnswerScore by their values thanks to ==, eql? and hash methods on our own:

irb(main):069:0> AnswerScore.new(123, 0) == AnswerScore.new(123, 0)
=> true
irb(main):070:0> AnswerScore.new(123, 0) == AnswerScore.new(123, 1)
=> false
irb(main):071:0> AnswerScore.new(123, 0) == BigDecimal("0")
=> false
irb(main):072:0> AnswerScore.new(123, 0) == AnswerScore.new(456, 0)
=> false

Same results will give us the .eql? operator since == is alias of it.

Adding two value objects

Ok, you can compare two objects, what now? And there's also an id, shouldn't this be an Entity? Nope, it shouldn't, we treat this id to distinguish scores of different skills. Adding two scores of two different skills wouldn't make much sense, right? Imagine adding money in dollars and pounds sterling without distinguishing the currency.

Let's implement + operator on the object then.

class AnswerScore
  def initialize(skill_id, score)
    @skill_id = skill_id
    @score = BigDecimal(score.to_s)
  end

  attr_reader :skill_id, :score

  def +(other)
    raise ArgumentError unless self.class === other
    raise ArgumentError if self.skill_id != other.skill_id

    score + other.score
  end

  def eql?(other)
    other.instance_of?(AnswerScore) && skill_id.eql?(other.skill_id) && score.eql?(other.score)
  end

  alias == eql?

  def hash
    AnswerScore.hash ^ [skill_id, score].hash
  end
end

And there it is, we won't be able to add anything wrong to our score:

# Same skills, different scores
irb(main):123:0> AnswerScore.new(123, 0) + AnswerScore.new(123, 1)
=> 0.1e1

# Different object
irb(main):124:0> AnswerScore.new(123, 0) + 5
Traceback (most recent call last):
        5: from /Users/fidel/.rbenv/versions/2.7.3/bin/irb:23:in `<main>'
        4: from /Users/fidel/.rbenv/versions/2.7.3/bin/irb:23:in `load'
        3: from /Users/fidel/.rbenv/versions/2.7.3/lib/ruby/gems/2.7.0/gems/irb-1.2.6/exe/irb:11:in `<top (required)>'
        2: from (irb):124
        1: from (irb):107:in `+'
ArgumentError (ArgumentError)

# Scores of different skills
irb(main):126:0> AnswerScore.new(123, 0) + AnswerScore.new(456, 1)
Traceback (most recent call last):
        5: from /Users/fidel/.rbenv/versions/2.7.3/bin/irb:23:in `<main>'
        4: from /Users/fidel/.rbenv/versions/2.7.3/bin/irb:23:in `load'
        3: from /Users/fidel/.rbenv/versions/2.7.3/lib/ruby/gems/2.7.0/gems/irb-1.2.6/exe/irb:11:in `<top (required)>'
        2: from (irb):124
        1: from (irb):107:in `+'
ArgumentError (ArgumentError)

Works great, but returns BigDecimal and we want to add more AnswerScore object to each other to cleanup and simplify our projection:

def calculate_scores(test_id, participant_id)
  RailsEventStore::Projection
    .from_stream(stream_name(test_id, participant_id))
    .init(-> { NullScore.new( })
    .when(
      SurveyExecution::AnswerRegistered,
      ->(state, event) do
        state += AnswerScore.new(
          skill_id: event.data.fetch(:skill_id),
          score: event.data.fetch(:score)
        )
      end
    )
    .run(Rails.configuration.event_store)
    .reduce(&:+)
    .average_score
end

This won't work, we don't have a NullScore, we should implement it:

class NullScore
  def +(other)
    raise ArgumentError unless AnswerScore === other

    other
  end

  def eql?(other)
    other.instance_of?(NullScore)
  end

  alias == eql?

  def hash
    NullScore.hash
  end
end

It just returns first real Value object, after addition. Great starting point for our projection than hacking internals of AnswerScore to provide that behaviour.

Be immutable

Getting back to the AnswerScore. We need to return a Value object from our AnswerScore rather than raw BigDecimal value. Adding two scores is no longer a score, we should return ScoreSum, probably.

class AnswerScore
  def initialize(skill_id, score)
    @skill_id = skill_id
    @score = BigDecimal(score.to_s)
  end

  attr_reader :skill_id, :score

  def +(other)
    raise ArgumentError unless self.class === other
    raise ArgumentError if self.skill_id != other.skill_id

    ScoreSum.new(skill_id: skill_id, sum: score + other.score, n: 2)
  end

  def average_score
    score.round(2)
  end

  def eql?(other)
    other.instance_of?(AnswerScore) && skill_id.eql?(other.skill_id) && score.eql?(other.score)
  end

  alias == eql?

  def hash
    AnswerScore.hash ^ [skill_id, score].hash
  end
end

class ScoreSum
  def initialize(skill_id:, sum:, n:)
    @skill_id = skill_id
    @sum = BigDecimal(sum.to_s)
    @n = Integer(n)
  end

  attr_reader :skill_id, :sum, :n

  def +(other)
    raise ArgumentError unless AnswerScore === other
    raise ArgumentError if self.skill_id != other.skill_id

    ScoreSum.new(sum: sum + other.score, skill_id: skill_id, n: n + 1)
  end

  def average_score
    (score / n).round(2)
  end

  def eql?(other)
    other.instance_of?(ScoreSum) && skill_id.eql?(other.skill_id) && sum.eql?(other.sum) && n.eql?(other.n)
  end

  alias == eql?

  def hash
    ScoreSum.hash ^ [skill_id, sum, n].hash
  end
end

How it rolls:

irb(main):254:0> AnswerScore.new(123, 0) + AnswerScore.new(123, 1)
=> #<ScoreSum:0x00000001137b3770 @skill_id=123, @sum=0.1e1, @n=2>
irb(main):255:0> AnswerScore.new(123, 0) + AnswerScore.new(123, 1) + AnswerScor
e.new(123, 1)
=> #<ScoreSum:0x0000000112030a30 @skill_id=123, @sum=0.2e1, @n=3>
irb(main):256:0> [AnswerScore.new(123, 0), AnswerScore.new(123, 1), AnswerScore
.new(123, 1)].reduce(&:+)
=> #<ScoreSum:0x00000001137a8938 @skill_id=123, @sum=0.2e1, @n=3>

What this gives us:

  • the objects are immutable, every time we do some operation, the new object is returned
  • we clearly explain our concept
  • we can incorporate specific behaviour for AnswerScore and ScoreSum, eg. average_score method which for score is simply a score, but for ScoreSum it's a sum divided by number of elements

Bad news

Our projection won't work now. Because current implementation in Rails Event Store framework doesn't allow that. Initial implementation worked because we used the Hash to maintain our state and we were mutating on and on the same instance of it😱

But there is light

WeDontDoThatHere = Class.new(StandardError)

def calculate_scores(test_id, participant_id)
  Rails
    .configuration
    .event_store
    .read
    .stream(stream_name(test_id, participant_id))
    .map do |event|
      case event.event_type
      when "SurveyExecution::AnswerRegistered"
        AnswerScore.new(skill_id: event.data.fetch(:skill_id), score: event.data.fetch(:score))
      else
        raise WeDontDoThatHere
      end
    end
    .reduce(&:+)
    .average_score
end

Does the same, and even looks less magical, at least to me. And the NullScore is obsolete now, we do map—reduce and there it is.

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Szymon Fiedler

I solve problems. This is the place where I share my thoughts on Software Engineering.

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