01Factory

FactoryGirl.define do
  factory :contract do
    sequence(:persnr) { |n| "#{n}" }
    sequence(:naam) { |n| "naam #{n}" }
    sequence(:voornaam) { |n| "voornaam #{n}" }
    sequence(:maand) { |n| n }
    sequence(:maand_teller) { |n| n }
    jaar 2015
  end  
end

02Test

require 'test_helper'

class ContractSerializerKoppenPerMaand < ActiveSupport::TestCase
  include FactoryGirl::Syntax::Methods
  
  attr_reader :contract
  
  setup do
    12.times do 
      create(:contract)
    end
    @stats = ContractSerializer.contracts_per_month
  end
  
  it "should retreive the data in the form of an array" do
    assert false
    assert_includes @stats.keys, :aantal_koppen
  end
  
end

03Model contract.rb

class Contract < ActiveRecord::Base
  
  scope :contracts_per_month, -> { select("jaar, maand, count(persnr) as aantal_koppen").group("maand_teller") }
  
  def self.serialize_for_graph
    ContractSerializer.contracts_per_month.to_json
  end
end

04serializer.rb

class ContractSerializer
  def self.contracts_per_month
    contracts = Contract.contracts_per_month
    
    debug = contracts.map { |c| ["#{c.jaar}/#{c.maand}", c.aantal_koppen] }
    
    byebug
  end
end

05Running test

~/workspace (master) $ rake test test/units/contract_serializer/koppen_per_maand.rb
Run options: --seed 32154

# Running:


[3, 12] in /home/ubuntu/workspace/app/lib/contract_serializer.rb
    3:     contracts = Contract.contracts_per_month
    4:     
    5:     debug = contracts.map { |c| ["#{c.jaar}/#{c.maand}", c.aantal_koppen] }
    6:     
    7:     byebug
=>  8:   end
    9: 
   10:   def self.koppen_per_maand_with_annotation
   11:     contracts = Contract.koppen_per_maand
   12: 

(byebug) contracts.last.inspect
#<Contract id: nil, jaar: 2015, maand: 12>
(byebug)