rails activesupport notifications - wrong db runtime value

Question

I\'m trying to log requests for my REST API application. I\'m using rails notifications for this , like here http://railscasts.com/episodes/249-notifications-in-rails-3

Answer 1

Ok, it seems like a bug. Let see, what's going on:

First of all, we have AR railtie for controller action and its implementation for setting db_runtime by using cleanup_view_runtime hook

def cleanup_view_runtime
      if ActiveRecord::Base.connected?
       db_rt_before_render = ActiveRecord::LogSubscriber.reset_runtime
       runtime = super
       db_rt_after_render = ActiveRecord::LogSubscriber.reset_runtime
       self.db_runtime = db_rt_before_render + db_rt_after_render
       runtime - db_rt_after_render
     else
       super
     end
end

App calls controller action -> action doing some db queries and renders some stuff -> before and after rendering AR Logger saves runtime data. Good.

Let see how works respond_with

def respond_with(*resources, &block)
  raise "In order to use respond_with, first you need to declare the formats your " <<
        "controller responds to in the class level" if self.class.mimes_for_respond_to.empty?

  if collector = retrieve_collector_from_mimes(&block)
    options = resources.size == 1 ? {} : resources.extract_options!
    options[:default_response] = collector.response
    (options.delete(:responder) || self.class.responder).call(self, resources, options)
  end
end

def self.call(*args)
  new(*args).respond
end

def to_format
  if get? || !has_errors? || response_overridden?
    default_render
  else
    display_errors
  end
rescue ActionView::MissingTemplate => e
  api_behavior(e)
end

It seems like too much code here, but you should see the callstack for this issue: respond_with -> self.class.responder.respond -> self.class.responder.to_format -> default_render -> default_renderer raise ActionView::MissingTemplate(because we don't have any). At this moment we can see implementation for rendering :json and :xml(api_behaviour) through catching ActionView::MissingTemplate.

Now we know how respond_with works, but AR Logger does'nt know.. cleanup_view_runtime hook is called twice: for default_renderer(at that time template data was prepared and some db queries were called, but we catch ActionView::MissingTemplate in rendering proccess)

db_rt_before_render = ActiveRecord::LogSubscriber.reset_runtime
runtime = super # <-- here
db_rt_after_render = ActiveRecord::LogSubscriber.reset_runtime

and for api_behavour(at that time all template data was ready for rendering and no db queries)

Some messy explanation, but I hope it will be helpful :)