In Rails, when you save an instance variable and the an exception occurs, the error reporting process calls `.inspect` on each instance variable, multiple times. This is extremely bad if you store objects whose representation is really expensive. An example is an Active Record scope matching a lot of objects (> thousands). The application simply…

application_controller.rb

class ApplicationController < ActionController::Base

  def index
    foo1 # creates the first instance variable
    foo2 # creates the second instance variable
    
    request.xxx # raises an exception
    
    # see the logs, you should expect `.inspect` being called several times for each instance variable
  end
  
  private
  
    def foo1
      @foo1 ||= Foo.new(:foo1)
    end
    
    def foo2
      @foo2 ||= Foo.new(:foo2)
    end

end

foo.rb

class Foo
  def initialize(name)
    @name = name
    log(:initialize)
  end

  def inspect
    log(:inspect)
  end

  def to_s
    log(:to_s)
  end

  def log(context)
    Rails.logger.debug "XXX Foo(#{@name}).#{context}"
  end
end