ruby - Rspec expectation for subclass method -

ruby - Rspec expectation for subclass method -

i want test code:

smtp = net::smtp.start(config[:server])

with spec:

expect(net::smtp).to receive(:start).with('testserver')

but take error:

failure/error: unable find matching line backtrace systemstackerror: stack level deep

what missing? should utilize fakeweb stubbing activity or expect functional plenty stage?

ruby testing rspec