# File activesupport/lib/active_support/testing/assertions.rb, line 175
def assert_changes(expression, message = nil, from: UNTRACKED, to: UNTRACKED, &block)
  exp = expression.respond_to?(:call) ? expression : -> { eval(expression.to_s, block.binding) }

  before = exp.call
  retval = _assert_nothing_raised_or_warn("assert_changes", &block)

  unless from == UNTRACKED
    error = "Expected change from #{from.inspect}"
    error = "#{message}.\n#{error}" if message
    assert from === before, error
  end

  after = exp.call

  error = "#{expression.inspect} didn't change"
  error = "#{error}. It was already #{to}" if before == to
  error = "#{message}.\n#{error}" if message
  refute_equal before, after, error

  unless to == UNTRACKED
    error = "Expected change to #{to}\n"
    error = "#{message}.\n#{error}" if message
    assert to === after, error
  end

  retval
end

Assertion that the result of evaluating an expression is changed before and after invoking the passed in block.

assert_changes 'Status.all_good?' do
  post :create, params: { status: { ok: false } }
end

You can pass the block as a string to be evaluated in the context of the block. A lambda can be passed for the block as well.

assert_changes -> { Status.all_good? } do
  post :create, params: { status: { ok: false } }
end

The assertion is useful to test side effects. The passed block can be anything that can be converted to string with to_s.

assert_changes :@object do
  @object = 42
end

The keyword arguments :from and :to can be given to specify the expected initial value and the expected value after the block was executed.

assert_changes :@object, from: nil, to: :foo do
  @object = :foo
end

An error message can be specified.

assert_changes -> { Status.all_good? }, 'Expected the status to be bad' do
  post :create, params: { status: { incident: true } }
end

# File activesupport/lib/active_support/testing/assertions.rb, line 86
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|
    error  = "#{code.inspect} didn't change by #{diff}"
    error  = "#{message}.\n#{error}" if message
    assert_equal(before_value + diff, exp.call, error)
  end

  retval
end

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 222
def assert_no_changes(expression, message = nil, from: UNTRACKED, &block)
  exp = expression.respond_to?(:call) ? expression : -> { eval(expression.to_s, block.binding) }

  before = exp.call
  retval = _assert_nothing_raised_or_warn("assert_no_changes", &block)

  unless from == UNTRACKED
    error = "Expected initial value of #{from.inspect}"
    error = "#{message}.\n#{error}" if message
    assert from === before, error
  end

  after = exp.call

  error = "#{expression.inspect} changed"
  error = "#{message}.\n#{error}" if message

  if before.nil?
    assert_nil after, error
  else
    assert_equal before, after, error
  end

  retval
end

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

assert_no_changes 'Status.all_good?' do
  post :create, params: { status: { ok: true } }
end

Provide the optional keyword argument :from to specify the expected initial value.

assert_no_changes -> { Status.all_good? }, from: true do
  post :create, params: { status: { ok: true } }
end

An error message can be specified.

assert_no_changes -> { Status.all_good? }, 'Expected the status to be good' do
  post :create, params: { status: { ok: false } }
end

# File activesupport/lib/active_support/testing/assertions.rb, line 137
def assert_no_difference(expression, message = nil, &block)
  assert_difference expression, 0, message, &block
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, params: { article: invalid_attributes }
end

A lambda can be passed in and evaluated.

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

An error message can be specified.

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

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

assert_no_difference [ 'Article.count', -> { Post.count } ] do
  post :create, params: { article: invalid_attributes }
end

# File activesupport/lib/active_support/testing/assertions.rb, line 21
def assert_not(object, message = nil)
  message ||= "Expected #{mu_pp(object)} to be nil or false"
  assert !object, message
end

Asserts 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'  # => Expected "foo" to be 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 33
def assert_nothing_raised
  yield
rescue => error
  raise Minitest::UnexpectedError.new(error)
end

Assertion that the block should not raise an exception.

Passes if evaluated code in the yielded block raises no exception.

assert_nothing_raised do
  perform_service(param: 'no_exception')
end