class BaseWorker

  def execute
    logger.info 'Starting the worker...'
  end

end

describe BaseWorker do

  it 'should log an info message' do
    base_worker = BaseWorker.new
    logger_mock = double('Logging::Rails').as_null_object
    Logging::Rails.stub_chain(:logger, :info).and_return(logger_mock)

    logger_mock.should_receive(:info).with('Starting the worker...')
    base_worker.execute
    Logging::Rails.unstub(:logger)
  end

end

Failure/Error: logger_mock.should_receive(:info).with('Starting worker...')
   (Double "Logging::Rails").info("Starting worker...")
       expected: 1 time
       received: 0 times

class BaseWorker

  attr_accessor :log

  def initialize
    @log = logger
  end

  def execute
    @log.info 'Starting the worker...'
  end

end

describe BaseWorker do
  it 'should log an info message' do
    base_worker = BaseWorker.new
    logger_mock = double('logger')
    base_worker.log = logger_mock

    logger_mock.should_receive(:info).with('Starting the worker...')
    base_worker.execute
  end
end

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

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

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

Rails.logger.error "Some useful error message"

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

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

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

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

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

expect { my_method }.to output("my message").to_stdout
expect { my_method }.to output("my error").to_stderr