assert_difference (Test::Unit::Assertions)

    # File lib/active_support/core_ext/test/unit/assertions.rb, line 37
          def assert_difference(expressions, difference = 1, message = nil, &block)
            expression_evaluations = Array(expressions).collect{ |expression| lambda { eval(expression, block.send!(:binding)) } }

            original_values = expression_evaluations.inject([]) { |memo, expression| memo << expression.call }
            yield
            expression_evaluations.each_with_index do |expression, i|
              assert_equal original_values[i] + difference, expression.call, message
            end
          end