RSpec: how to test Rails logger message expectations?

While I agree you generally don't want to test loggers, there are times it may be useful.

I have had success with expectations on Rails.logger.

Using RSpec's deprecated should syntax:

Rails.logger.should_receive(:info).with("some message")

Using RSpec's newer expect syntax:

expect(Rails.logger).to receive(:info).with("some message")

Note: In controller and model specs, you have to put this line before the message is logged. If you put it after, you'll get an error message like this:

Failure/Error: expect(Rails.logger).to receive(:info).with("some message")
       (#<ActiveSupport::Logger:0x007f27f72136c8>).info("some message")
           expected: 1 time with arguments: ("some message")
           received: 0 times

With RSpec 3+ version

Actual code containing single invocation of Rails.logger.error:

Rails.logger.error "Some useful error message"

Spec code:

expect(Rails.logger).to receive(:error).with(/error message/)

If you want the error message to be actually logged while the spec runs then use following code:

expect(Rails.logger).to receive(:error).with(/error message/).and_call_original

Actual code containing multiple invocations of Rails.logger.error:

Rails.logger.error "Technical Error Message"
Rails.logger.error "User-friendly Error Message"

Spec code:

expect(Rails.logger).to receive(:error).ordered
expect(Rails.logger).to receive(:error).with(/User-friendly Error /).ordered.and_call_original

Also if you care about just matching the first message and not any subsequent messages then you can use following

  expect(Rails.logger).to receive(:debug).with("Technical Error Message").ordered.and_call_original
  expect(Rails.logger).to receive(:debug).at_least(:once).with(instance_of(String)).ordered

Note in above variation setting .ordered is important else expectations set start failing.

References:

http://www.relishapp.com/rspec/rspec-mocks/v/3-4/docs/setting-constraints/matching-arguments

http://www.relishapp.com/rspec/rspec-mocks/v/3-4/docs/setting-constraints/message-order