1 files changed, 43 insertions, 0 deletions
diff --git a/activesupport/lib/active_support/whiny_nil.rb b/activesupport/lib/active_support/whiny_nil.rb
new file mode 100644
index 0000000000..4a83cd585e
--- /dev/null
+++ b/activesupport/lib/active_support/whiny_nil.rb
@@ -0,0 +1,43 @@
+# Extensions to nil which allow for more helpful error messages for 
+# people who are new to rails.
+#
+# The aim is to ensure that when users pass nil to methods where that isn't
+# appropriate, instead of NoMethodError and the name of some method used
+# by the framework users will see a message explaining what type of object 
+# was expected.
+
+
+class NilClass
+  WHINERS = [ ActiveRecord::Base, Array ]
+  
+  @@method_class_map = Hash.new
+  
+  WHINERS.each do |klass|
+    methods = klass.public_instance_methods - public_instance_methods
+    methods.each do |method|
+      @@method_class_map[method.to_sym] = klass
+    end
+  end
+
+  private
+    def method_missing(method, *args, &block)
+      if @@method_class_map.include?(method)
+        raise_nil_warning_for @@method_class_map[method]
+      else
+        super
+      end
+    end
+
+    def raise_nil_warning_for(klass)
+      raise NoMethodError, NIL_WARNING_MESSAGE % klass
+    end
+
+    NIL_WARNING_MESSAGE = <<-end_message unless const_defined?(:NIL_WARNING_MESSAGE)
+WARNING:  You have a nil object when you probably didn't expect it!  Odds are you
+want an instance of %s instead.
+
+Look in the callstack to see where you're working with an object that could be nil.
+Investigate your methods and make sure the object is what you expect!
+    end_message
+end
+