mylar_logger.rb

class MylarLogger
  attr_accessor :logger, :log, :saved_lines

  def initialize logger, saved_lines: 20
    @logger, @log, @saved_lines = logger, [], saved_lines
  end

  def method_missing name, *args, &block
    @log.push({ method: name, args: args })
    @log.shift if @log.length > @saved_lines

    @logger.send name, *args, &block
  end

  def has_logged? regex
    regex = Regexp.compile(Regexp.quote(regex)) if regex.kind_of? String

    regex =~ @log.map{ |e| e[:args] }.flatten.join(" ")
  end

  def respond_to_missing? name, include_all=false
    @logger.respond_to? name, include_all
  end
end

module MylarLoggerHelper
  def assert_logged regex
    yield if block_given?

    assert Rails.logger.has_logged?(regex), <<STRING
Expected:
  #{ regex }
To have been logged. Only logged:
  #{ Rails.logger.log.join("\n") }
STRING
  end
end

Rails.logger = MylarLogger.new Rails.logger