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        # Ruby 1.9.2: disallow explicit coercion via method_missing.
  47        if method == :to_ary || method == :to_str
  48          super
  49        elsif klass = METHOD_CLASS_MAP[method]
  50          raise_nil_warning_for klass, method, caller
  51        else
  52          super
  53        end
  54      end
  55 
  56      # Raises a NoMethodError when you attempt to call a method on +nil+.
  57      def raise_nil_warning_for(class_name = nil, selector = nil, with_caller = nil)
  58        message = "You have a nil object when you didn't expect it!"
  59        message << "\nYou might have expected an instance of #{class_name}." if class_name
  60        message << "\nThe error occurred while evaluating nil.#{selector}" if selector
  61 
  62        raise NoMethodError, message, with_caller || caller
  63      end
  64  end