Delegate missing constants to object class via const_missing

lib/draper/automatic_delegation.rb

@@ -49,6 +49,13 @@ def respond_to_missing?(method, include_private = false)
         super || delegatable?(method)
       end
 
+      # Proxies missing constants to the source class.
+      def const_missing(name)
+        return object_class.const_get(name) if object_class?
+
+        super
+      end
+
       # @private
       def delegatable?(method)
         object_class? && object_class.respond_to?(method)

spec/draper/decorator_spec.rb

@@ -722,6 +722,30 @@ def hello_world
         end
       end
 
+      context ".const_missing" do
+        context "without an object class" do
+          it "raises a NameError on missing constants" do
+            expect{Decorator::HELLO_WORLD}.to raise_error NameError, /HELLO_WORLD/
+          end
+        end
+
+        context "with an object class" do
+          it "delegates constants that exist on the object class" do
+            object_class = Class.new
+            object_class.const_set(:HELLO_WORLD, :delegated)
+            allow(Decorator).to receive_messages object_class: object_class
+
+            expect(Decorator::HELLO_WORLD).to be :delegated
+          end
+
+          it "raises a NameError for constants that do not exist on the object class" do
+            allow(Decorator).to receive_messages object_class: Class.new
+
+            expect{Decorator::HELLO_WORLD}.to raise_error NameError, /HELLO_WORLD/
+          end
+        end
+      end
+
       describe "#respond_to?" do
         it "returns true for its own methods" do
           Decorator.class_eval{def hello_world; end}