test.rb

I am assuming that the modles will look like this

class Customer < ApplicationRecord
  has_many :purcases
  # Rest of the code
end

class Purcase < ApplicationRecord
  has_one :discount_coupon_usage
  # Rest of the code
end

class DiscountCouponUsage < ApplicationRecord
  belongs_to :purcase
  belongs_to :discount_coupon
  # Rest of the code
end

class DiscountCoupon < ApplicationRecord
  has_many :discount_coupon_usages
  # Rest of the code
end

And here is the test
class CustomerTest << ActiveSupport::TestCase
  test "a new customer will get a 15% discount on all your purchases today" do
    customer = customer(:one)
    purcases = purcase(:three)
    customer << purcases
    customer.purcases.each do |purcase|
      assert_equal(purcase.total_paid, purcase.total_purcase - (purcase.total_purcase * 15 / 100))
    end
  end

  test "existing customer and has hold a loyalty card, will get a 10% discount" do
    customer  = customer(:one)
    customer.update(created_at: 3.months.ago, own_loyalty_card: true)
    customer.purcase = purcase(:one)
    assert_equal(customer.purcase.total_paid, purcase.total_purcase - (purcase.total_purcase * 15 / 100))
  end

  test "existing customer and without a loyalty card, will not get a discount" do
    customer  = customer(:one)
    customer.update(created_at: 3.months.ago, own_loyalty_card: true)
    customer.purcase = purcase(:one)
    assert_equal(customer.purcase.total_paid, purcase.total_purcase)
  end

  test "a new customer with discout coupon will get 20% off" do
    customer  = customer(:one)
    customer.purcase = purcase(:one)
    discoun_coupon = discount_coupon(:one)
    discoun_coupn_usage = DiscountCouponUsage.create(purcase_id:  customer.purcase.id, discoun_coupon_id: discoun_coupon.id)
    assert_equal(customer.purcase.total_paid, purcase.total_purcase - (purcase.total_purcase * 20 / 100))
  end
end