vendor/rails/activesupport/lib/active_support/whiny_nil.rb

   1 # Extensions to +nil+ which allow for more helpful error messages for people who
   2 # are new to Rails.
   3 #
   4 # Ruby raises NoMethodError if you invoke a method on an object that does not
   5 # respond to it:
   6 #
   7 #   $ ruby -e nil.destroy
   8 #   -e:1: undefined method `destroy' for nil:NilClass (NoMethodError)
   9 #
  10 # With these extensions, if the method belongs to the public interface of the
  11 # classes in NilClass::WHINERS the error message suggests which could be the
  12 # actual intended class:
  13 #
  14 #   $ script/runner nil.destroy
  15 #   ...
  16 #   You might have expected an instance of ActiveRecord::Base.
  17 #   ...
  18 #
  19 # NilClass#id exists in Ruby 1.8 (though it is deprecated). Since +id+ is a fundamental
  20 # method of Active Record models NilClass#id is redefined as well to raise a RuntimeError
  21 # and warn the user. She probably wanted a model database identifier and the 4
  22 # returned by the original method could result in obscure bugs.
  23 #
  24 # The flag <tt>config.whiny_nils</tt> determines whether this feature is enabled.
  25 # By default it is on in development and test modes, and it is off in production
  26 # mode.
  27 class NilClass
  28   WHINERS = [::Array]
  29   WHINERS << ::ActiveRecord::Base if defined? ::ActiveRecord
  30
  31   METHOD_CLASS_MAP = Hash.new
  32
  33   WHINERS.each do |klass|
  34     methods = klass.public_instance_methods - public_instance_methods
  35     class_name = klass.name
  36     methods.each { |method| METHOD_CLASS_MAP[method.to_sym] = class_name }
  37   end
  38
  39   # Raises a RuntimeError when you attempt to call +id+ on +nil+.
  40   def id
  41     raise RuntimeError, "Called id for nil, which would mistakenly be 4 -- if you really wanted the id of nil, use object_id", caller
  42   end
  43
  44   private
  45     def method_missing(method, *args, &block)
  46       raise_nil_warning_for METHOD_CLASS_MAP[method], method, caller
  47     end
  48
  49     # Raises a NoMethodError when you attempt to call a method on +nil+.
  50     def raise_nil_warning_for(class_name = nil, selector = nil, with_caller = nil)
  51       message = "You have a nil object when you didn't expect it!"
  52       message << "\nYou might have expected an instance of #{class_name}." if class_name
  53       message << "\nThe error occurred while evaluating nil.#{selector}" if selector
  54
  55       raise NoMethodError, message, with_caller || caller
  56     end
  57 end
  58