An RSpec custom matcher to test code that logs information into log files.

Writing logs is an easy way to store any kind of information for further analysis later on. It's commonly used to store analytics events and then make the logs a data source for data engineering tasks. This matcher makes application-logging-testing easier.

Add this line to your application's Gemfile:

group :test do
  gem 'rspec-log_matcher'
end

And then execute:

$ bundle install

Lastly, in your spec_helper.rb (or rails_helper.rb) add the following line inside the configuration block:

RSpec.configure do |config|
  # [snip]

  RSpec::LogMatcher.configure!(config)
end

# app/services/payment_service.rb
class PaymentService
  def self.call
    # [snip]

    logger.info "event=payment-successful properties=#{data.to_json}"
  end
end

# spec/services/payment_service_spec.rb
require 'spec_helper'

RSpec.describe PaymentService do
  describe '.call' do
    subject { described_class.call }

    it 'logs event information' do
      expect { subject }.to log("event=payment-successful properties=#{build_expected_json}")
    end
  end
end

# spec/requests/users_spec.rb
require 'spec_helper'

RSpec.describe 'Users' do
  describe 'GET /index' do
    expect { get(users_path) }.to log('Page view - Users index')
  end
end

# spec/features/sign_in_spec.rb
require 'spec_helper'

RSpec.feature 'Sign in' do
  scenario 'successful sign in' do
    user = create(:user)

    visit sign_in_path
    fill_form(user)
    submit_form

    expect(page).to have_text('Welcome!')
    expect(page).to log("User #{user.id} has logged in")
  end
end

Regular expressions and procs are also valid object types for the expected logs, for more use cases refer to the spec file.

The default path for the log file is log/test.log. It can be configured via an environment variable called LOG_PATH.

This is useful when tests are run parallely, and each process has their own log file.

The matcher reads into the log file and looks for the expected logs to be present in the log file.

When the subject is a proc, the matcher will execute proc and compare against the logs introduced by the proc execution.

When the subject is a Capybara::Session (from a feature spec, system tests), the matcher will store the position in the file to the last byte in a before hook. Then, when the example is run, it will compare against the changes introduced by the example using the position stored as the beginning of the logs.

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

Bug reports and pull requests are welcome on GitHub at https://github.com/juanmanuelramallo/rspec-log_matcher. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the code of conduct.

The gem is available as open source under the terms of the MIT License.

Everyone interacting in the Rspec::LogMatcher project's codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.