title: What's the impact of eager_load=true?
body: <p>I need to know why <code>eager_load</code> is preferred to be <code>false</code> in non-production environments? One of the arguments that I have heard of says that <code>eager_load</code> eager loads most of Rails and application in memory. Hence using eager_load for individual test makes it run slower. However this raises some questions like how does a test run without loading Rails and application related code? What is the Rails and application related code that is being eager loaded? <code>config.eager_load_namespaces</code> gives the following classes:</p>
  <pre><code>ActiveSupport
  ActionDispatch
  ActiveModel
  ActionView
  ActionController
  ActiveRecord
  ActionMailer
  Jquery::Rails::Engine
  MyApp::Application
  </code></pre>
  <p>Are all of these classes and their subclasses being eager loaded?</p>
  <p>What are the clear disadvantages of using <code>eager_load = false</code>  in development or testing environment?</p>