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 error message can be specified.

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

[Source]

    # File vendor/rails/activesupport/lib/active_support/testing/core_ext/test/unit/assertions.rb, line 38
38:       def assert_difference(expressions, difference = 1, message = nil, &block)
39:         expression_evaluations = Array(expressions).map do |expression|
40:           [expression, lambda do
41:             eval(expression, block.__send__(:binding))
42:           end]
43:         end
44: 
45:         original_values = expression_evaluations.inject([]) { |memo, expression| memo << expression[1].call }
46:         yield
47:         expression_evaluations.each_with_index do |expression, i|
48:           full_message = ""
49:           full_message << "#{message}.\n" if message
50:           full_message << "<#{expression[0]}> was the expression that failed"
51:           assert_equal original_values[i] + difference, expression[1].call, full_message
52:         end
53:       end

Assertion that the numeric result of evaluating an expression is not changed before and after invoking the passed in block.

  assert_no_difference 'Article.count' do
    post :create, :article => invalid_attributes
  end

A error message can be specified.

  assert_no_difference 'Article.count', "An Article should not be destroyed" do
    post :create, :article => invalid_attributes
  end

[Source]

    # File vendor/rails/activesupport/lib/active_support/testing/core_ext/test/unit/assertions.rb, line 67
67:       def assert_no_difference(expressions, message = nil, &block)
68:         assert_difference expressions, 0, message, &block
69:       end