test "that rate with same from- and to-rate is invalid" do
    rate = ExchangeRate.new
    rate.name = "From To Same"
    euro = Currency.new
    rate.from_currency = euro
    rate.to_currency = euro

    assert rate.from_currency == rate.to_currency

    assert !rate.valid?
  end


Message:

  1) Failure:
test_that_rate_with_same_from-_and_to-rate_is_invalid(ExchangeRateTest) [/test/unit/exchange_rate_test.rb:25]:
<false> is not true.

exchange_rate.rb:

class ExchangeRate < ActiveRecord::Base

  validates_presence_of :name, :from_currency, :to_currency
  validates_difference_of :from_currency, :to_currency

  has_many :quotes, :as => :object_with_quotes

  has_one :latest_quote, :as => :object_with_quotes, :class_name => 'Quote',
                          :order => "datetime DESC", :readonly => true

  belongs_to :from_currency, :class_name => "Currency"
  belongs_to :to_currency, :class_name => "Currency"
end