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, :article => {...}
end

An arbitrary expression is passed in and evaluated.

assert_difference 'assigns(:article).comments(:reload).size' do
  post :create, :comment => {...}
end

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

assert_difference 'Article.count', -1 do
  post :delete, :id => ...
end

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

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

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

assert_difference lambda { Article.count }, 2 do
  post :create, :article => {...}
end

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

A error message can be specified.

assert_difference 'Article.count', -1, "An Article should be destroyed" do
  post :delete, :id => ...
end

# File activesupport/lib/active_support/testing/assertions.rb, line 47
      def assert_difference(expression, difference = 1, message = nil, &block)
        expressions = Array.wrap expression

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

        yield

        expressions.zip(exps).each_with_index do |(code, e), i|
          error  = "#{code.inspect} didn't change by #{difference}"
          error  = "#{message}.\n#{error}" if message
          assert_equal(before[i] + difference, e.call, error)
        end
      end