Delegate missing constants to object class via const_missing

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Draper Changelog
 
+## Unreleased
+
+### New Features
+* Delegate missing constants to the object class via `const_missing` when using `delegate_all` [#955](https://github.com/drapergem/draper/pull/955)
+
 ## 4.0.6 - 2025-11-15
 
 ### Fixes

README.md

@@ -547,6 +547,21 @@ the decorator will first try to call the method on the parent decorator class. I
 the method does not exist on the parent decorator class, it will then try to call
 the method on the decorated `object`. This is a very permissive interface.
 
+Constants defined on the source model class are also accessible via the decorator
+class when using `delegate_all`:
+
+```ruby
+class Car
+  WHEELS_COUNT = 4
+end
+
+class CarDecorator < ApplicationDecorator
+  delegate_all
+end
+
+CarDecorator::WHEELS_COUNT # => 4
+```
+
 If you want to strictly control which methods are called within views, you can
 choose to only delegate certain methods from the decorator to the source model:
 

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? && object_class.const_defined?(name)
+
+        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}