Why can't New Relic display some metrics for a Ruby on Rails action?

I have a Rails action that has 83% of its run time spent on this:

ApplicationCode in (Nested/Controller/my_controller/my_action

There's an info popup that says this:

Deeper visibility is not available because these classes and methods are not instrumented with the Ruby Agents current configuration. Consult our documentation to find out how to add custom metrics to your app.

I'm not doing anything unusual in this action. Any ideas why it might say this? Is there some internal Rails code that's not being traced?

New Relic agent is not a full profiler. It is able to trace some methods it knows are available in the many frameworks and libraries it supports.

Usually when you see an entry like that (ApplicationCode in (Nested/Controller/my_controller/my_action) it means that either it is code inside that method that is taking too long (think a long running loop) or some of your methods that the agent wouldn't know.

class MyController < ApplicationController
  def my_action
    1000.times do
      # something that takes time
    end
  end

  def another_action
    Foo.new.do_something
  end
end

class Foo
  def do_something
    # something that takes time
  end
end

In my_action you could use the Trace API to trace blocks of code.

require 'new_relic/agent/method_tracer'

class MyController < ApplicationController
  extend ::NewRelic::Agent::MethodTracer
  def my_action
    self.class.trace_execution_scoped(['Custom/slow_action/beginning_work']) do
      1000.times do
        # something that takes time
      end
    end
  end
end

Or for the case that you are calling another method (like in another_action), you can use method tracers.

require 'new_relic/agent/method_tracer'

class Foo
  def do_something
    # something that takes time
  end

  class << self
    include ::NewRelic::Agent::MethodTracer

    add_method_tracer :do_something, 'Custom/do_something'
  end
end