module ActiveSupport::Testing::Assertions

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

An error message can be specified.

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

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

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

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

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

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

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

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

An arbitrary expression is passed in and evaluated.

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

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

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(&:call)
  retval = 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
  retval
end

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

An error message can be specified.

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

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

def assert_no_difference(expression, message = nil, &block)
  assert_difference expression, 0, message, &block
end

assert_not foo, 'foo should be false'

An error message can be specified.

assert_not 'foo' # => Expected "foo" to be nil or false
assert_not false # => true
assert_not nil # => true

like if foo.
+nil+ or +false+. "Truthy" means "considered true in a conditional"
Asserts that an expression is not truthy. Passes if object is

def assert_not(object, message = nil)
  message ||= "Expected #{mu_pp(object)} to be nil or false"
  assert !object, message
end