Provide a way decorate_attribute for attribute type decoration

activerecord/lib/active_record/attributes.rb

@@ -266,6 +266,12 @@ def load_schema! # :nodoc:
         end
       end
 
+      def decorate_attribute(name, type_name, default: NO_DEFAULT_PROVIDED, **options)
+        attribute(name, default: default) do |subtype|
+          Type.lookup(type_name, **options, subtype: subtype, adapter: Type.adapter_name_from(self))
+        end
+      end
+
       private
         NO_DEFAULT_PROVIDED = Object.new # :nodoc:
         private_constant :NO_DEFAULT_PROVIDED

activerecord/test/cases/serialized_attribute_test.rb

@@ -424,6 +424,24 @@ def changed_in_place?(old, new)
     end
   end
 
+  def test_decorate_attribute_on_serialized_attribute
+    old_registry = ActiveRecord::Type.registry
+    ActiveRecord::Type.registry = ActiveRecord::Type.registry.dup
+    ActiveRecord::Type.register :encrypted, EncryptedType
+
+    klass = Class.new(ActiveRecord::Base) do
+      self.table_name = Topic.table_name
+      store :content
+      decorate_attribute :content, :encrypted
+    end
+
+    topic = klass.create!(content: { trial: true })
+
+    assert_equal({ "trial" => true }, topic.content)
+  ensure
+    ActiveRecord::Type.registry = old_registry
+  end
+
   def test_decorated_type_with_type_for_attribute
     old_registry = ActiveRecord::Type.registry
     ActiveRecord::Type.registry = ActiveRecord::Type.registry.dup