From 22b31931b179584de4f61634d08ee0ad0ce1a1a7 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 27 Feb 2026 13:00:50 +0000
Subject: [PATCH 1/4] Initial plan


From b4473db8c4b03723cba8a8d148bc9e4664eebf19 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 27 Feb 2026 13:08:33 +0000
Subject: [PATCH 2/4] Add const_missing to delegate missing constants to object
 class

Co-authored-by: Alexander-Senko <1022687+Alexander-Senko@users.noreply.github.com>
---
 lib/draper/automatic_delegation.rb |  7 +++++++
 spec/draper/decorator_spec.rb      | 24 ++++++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/lib/draper/automatic_delegation.rb b/lib/draper/automatic_delegation.rb
index 0546f4c4..b7aca0b9 100644
--- a/lib/draper/automatic_delegation.rb
+++ b/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)
diff --git a/spec/draper/decorator_spec.rb b/spec/draper/decorator_spec.rb
index 5536c696..4bf181cd 100644
--- a/spec/draper/decorator_spec.rb
+++ b/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}

From a2423d2c21aac71971f51d9fdde9637e1b35fd4a Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 27 Feb 2026 13:16:16 +0000
Subject: [PATCH 3/4] Update README and CHANGELOG for const_missing constant
 delegation

Co-authored-by: Alexander-Senko <1022687+Alexander-Senko@users.noreply.github.com>
---
 CHANGELOG.md |  5 +++++
 README.md    | 15 +++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a4449c35..0934f409 100644
--- a/CHANGELOG.md
+++ b/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
diff --git a/README.md b/README.md
index 588434b3..271afc47 100644
--- a/README.md
+++ b/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:
 

From 04bd5726373d94556e26a742e8aa1e1372a3f9a8 Mon Sep 17 00:00:00 2001
From: Alexander Senko <Alexander.Senko@gmail.com>
Date: Sat, 28 Feb 2026 18:07:01 +0700
Subject: [PATCH 4/4] Update lib/draper/automatic_delegation.rb

Co-authored-by: y-yagi <yuuji.yaginuma@gmail.com>
---
 lib/draper/automatic_delegation.rb | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/draper/automatic_delegation.rb b/lib/draper/automatic_delegation.rb
index b7aca0b9..97f2310f 100644
--- a/lib/draper/automatic_delegation.rb
+++ b/lib/draper/automatic_delegation.rb
@@ -51,7 +51,7 @@ def respond_to_missing?(method, include_private = false)
 
       # Proxies missing constants to the source class.
       def const_missing(name)
-        return object_class.const_get(name) if object_class?
+        return object_class.const_get(name) if object_class? && object_class.const_defined?(name)
 
         super
       end