module ActiveSupport::Testing::Assertions

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

An error message can be specified.

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

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

def assert_no_changes(expression, message = nil, &block)
  exp = expression.respond_to?(:call) ? expression : -> { eval(expression.to_s, block.binding) }
  before = exp.call
  retval = yield
  after = exp.call
  error = "#{expression.inspect} did change to #{after}"
  error = "#{message}.\n#{error}" if message
  assert before == after, error
  retval
end