class_attribute gets instance methods which delegate to but may override their class values as you'd expect. Disable instance writer methods with :instance_writer => false.

activesupport/lib/active_support/core_ext/class/attribute.rb

@@ -24,14 +24,35 @@ class Class
   # For convenience, a query method is defined as well:
   #
   #   Subclass.setting?           # => false
+  #
+  # Instances may overwrite the class value in the same way:
+  #
+  #   Base.setting = true
+  #   object = Base.new
+  #   object.setting          # => true
+  #   object.setting = false
+  #   object.setting          # => false
+  #   Base.setting            # => true
+  #
+  # To opt out of the instance writer method, pass :instance_writer => false.
+  #
+  #   object.setting = false  # => NoMethodError
   def class_attribute(*attrs)
+    instance_writer = !attrs.last.is_a?(Hash) || attrs.pop[:instance_writer]
+
     s = singleton_class
     attrs.each do |attr|
       s.send(:define_method, attr) { }
-      s.send(:define_method, "#{attr}?") { !!send(attr) }
-      s.send(:define_method, "#{attr}=") do |value|
+      s.send(:define_method, :"#{attr}?") { !!send(attr) }
+      s.send(:define_method, :"#{attr}=") do |value|
         singleton_class.send(:define_method, attr) { value }
       end
+
+      define_method(attr) { self.class.send(attr) }
+      define_method(:"#{attr}?") { !!send(attr) }
+      define_method(:"#{attr}=") do |value|
+        singleton_class.send(:define_method, attr) { value }
+      end if instance_writer
     end
   end
 end

activesupport/test/core_ext/class/attribute_test.rb

@@ -2,13 +2,6 @@ require 'abstract_unit'
 require 'active_support/core_ext/class/attribute'
 
 class ClassAttributeTest < ActiveSupport::TestCase
-  class Base
-    class_attribute :setting
-  end
-
-  class Subclass < Base
-  end
-
   def setup
     @klass = Class.new { class_attribute :setting }
     @sub = Class.new(@klass)
@@ -40,8 +33,30 @@ class ClassAttributeTest < ActiveSupport::TestCase
     assert_equal true, @klass.setting?
   end
 
-  test 'no instance delegates' do
-    assert_raise(NoMethodError) { @klass.new.setting }
-    assert_raise(NoMethodError) { @klass.new.setting? }
+  test 'instance reader delegates to class' do
+    assert_nil @klass.new.setting
+
+    @klass.setting = 1
+    assert_equal 1, @klass.new.setting
+  end
+
+  test 'instance override' do
+    object = @klass.new
+    object.setting = 1
+    assert_nil @klass.setting
+    @klass.setting = 2
+    assert_equal 1, object.setting
+  end
+
+  test 'instance query' do
+    object = @klass.new
+    assert_equal false, object.setting?
+    object.setting = 1
+    assert_equal true, object.setting?
+  end
+
+  test 'disabling instance writer' do
+    object = Class.new { class_attribute :setting, :instance_writer => false }.new
+    assert_raise(NoMethodError) { object.setting = 'boom' }
   end
 end