assert_difference

Test numeric difference between the return value of an expression as a result of what is evaluated in the yielded block.

assert_difference 'Article.count' do
  post :create, params: { article: {...} }
end

An arbitrary expression is passed in and evaluated.

assert_difference 'Article.last.comments(:reload).size' do
  post :create, params: { comment: {...} }
end

An arbitrary positive or negative difference can be specified. The default is 1.

assert_difference 'Article.count', -1 do
  post :delete, params: { id: ... }
end

An array of expressions can also be passed in and evaluated.

assert_difference [ 'Article.count', 'Post.count' ], 2 do
  post :create, params: { article: {...} }
end

A hash of expressions/numeric differences can also be passed in and evaluated.

assert_difference ->{ Article.count } => 1, ->{ Notification.count } => 2 do
  post :create, params: { article: {...} }
end

A lambda or a list of lambdas can be passed in and evaluated:

assert_difference ->{ Article.count }, 2 do
  post :create, params: { article: {...} }
end

assert_difference [->{ Article.count }, ->{ Post.count }], 2 do
  post :create, params: { article: {...} }
end

An error message can be specified.

assert_difference 'Article.count', -1, 'An Article should be destroyed' do
  post :delete, params: { id: ... }
end

# File activesupport/lib/active_support/testing/assertions.rb, line 101
      def assert_difference(expression, *args, &block)
        expressions =
          if expression.is_a?(Hash)
            message = args[0]
            expression
          else
            difference = args[0] || 1
            message = args[1]
            Array(expression).index_with(difference)
          end

        exps = expressions.keys.map { |e|
          e.respond_to?(:call) ? e : lambda { eval(e, block.binding) }
        }
        before = exps.map(&:call)

        retval = _assert_nothing_raised_or_warn("assert_difference", &block)

        expressions.zip(exps, before) do |(code, diff), exp, before_value|
          actual = exp.call
          error  = "#{code.inspect} didn't change by #{diff}, but by #{actual - before_value}"
          error  = "#{message}.\n#{error}" if message
          assert_equal(before_value + diff, actual, error)
        end

        retval
      end