shoulda-matchers fail when attribute saved in a model's callback - ruby-on-rails

Question

class StudentPiggyBank < ActiveRecord::Base
PERIODS = [['tydzień', :week], ['miesiąc', :month], ['trzy miesiące', :three_months]]
RATES_MULTIPLIERS = {week: 1, month: 1.5, three_months: 2}
INTEREST_RATE_PRECISION = 2
before_validation :set_interest_rate
validates :completion_date, presence: true
validates :balance, numericality: {greater_than_or_equal_to: 0,
message: I18n.t('errors.messages.negative_piggy_bank_balance')}
validates :interest_rate, numericality: {greater_than_or_equal_to: 0,
message: I18n.t('errors.messages.negative_interest_rate')}
def self.date_from_param(period_param)
case period_param
when 'week'
1.week.from_now
when 'month'
1.month.from_now
when 'three_months'
3.months.from_now
end
end
protected
def set_interest_rate
num_of_days = completion_date - Date.today
if num_of_days >= 90
self.interest_rate = student.base_interest_rate.mult(RATES_MULTIPLIERS[:three_months], INTEREST_RATE_PRECISION)
elsif num_of_days >= 30
self.interest_rate = student.base_interest_rate.mult(RATES_MULTIPLIERS[:month], INTEREST_RATE_PRECISION)
else
self.interest_rate = student.base_interest_rate.mult(RATES_MULTIPLIERS[:week], INTEREST_RATE_PRECISION)
end
end
end
This code works. However, when testing with shoulda-matchers
describe StudentPiggyBank do
it { should validate_numericality_of(:interest_rate).is_greater_than_or_equal_to(0) }
it { should validate_numericality_of(:balance).is_greater_than_or_equal_to(0) }
end
I get errors for the line num_of_days = completion_date - Date.today:
NoMethodError:
undefined method `-' for nil:NilClass
Why completion_date is nil?

Answer 1

Well, it will basically do a described_class.new, so you won't have a completion_date. You can fix it like this:
describe StudentPiggyBank do
context 'with a completion date' do
before { subject.completion_date = 7.days.from_now }
it { should validate_numericality_of(:interest_rate).is_greater_than_or_equal_to(0) }
end
end