# File lib/active_support/testing/assertions.rb, line 34 def assert_difference(expression, difference = 1, message = nil, &block) b = block.send(:binding) exps = Array.wrap(expression) before = exps.map { |e| eval(e, b) } yield exps.each_with_index do |e, i| error = "#{e.inspect} didn't change by #{difference}" error = "#{message}.\n#{error}" if message assert_equal(before[i] + difference, eval(e, b), error) end end