module ActiveSupport::Testing::Assertions

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

An error message can be specified.

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

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

end
@object = 42
assert_changes :@object do

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

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

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

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

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

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