Trying to test scope with factorygirl

01contract.rb

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

02contract_serializer.rb

# lib/contract_serializer.rb
def self.koppen_per_maand(contract)
  contracts = contract.koppen_per_maand
  
  contracts.map { |c| ["#{c.jaar}/#{c.maand}", c.aantal_koppen] }
end

03test

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

04full error trace

# Running:

E

Finished in 0.167948s, 5.9542 runs/s, 0.0000 assertions/s.

  1) Error:
ContractSerializerKoppenPerMaand#test_0001_should retreive the data in the form of an array:
NoMethodError: undefined method `koppen_per_maand' for #<Array:0x00000006277350>
    app/lib/contract_serializer.rb:3:in `koppen_per_maand'
    test/units/contract_serializer/koppen_per_maand.rb:10:in `block in <class:ContractSerializerKoppenPerMaand>'