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

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

An 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 63
def assert_difference(expression, difference = 1, message = nil, &block)
  expressions = Array(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

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

An error message can be specified.

assert_no_difference 'Article.count', 'An Article should not be created' do
  post :create, article: invalid_attributes
end

# File activesupport/lib/active_support/testing/assertions.rb, line 92
def assert_no_difference(expression, message = nil, &block)
  assert_difference expression, 0, message, &block
end

Assert that an expression is not truthy. Passes if object is nil or false. “Truthy” means “considered true in a conditional” like if foo.

assert_not nil    # => true
assert_not false  # => true
assert_not 'foo'  # => 'foo' is not nil or false

An error message can be specified.

assert_not foo, 'foo should be false'

# File activesupport/lib/active_support/testing/assertions.rb, line 17
def assert_not(object, message = nil)
  message ||= "Expected #{mu_pp(object)} to be nil or false"
  assert !object, message
end