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 } => 1, ->{ Notification.count } => 2 do

A hash of expressions/numeric differences can also 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, *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