diff --git a/lib/dm-core/model.rb b/lib/dm-core/model.rb
index f1227bbd..df751119 100644
--- a/lib/dm-core/model.rb
+++ b/lib/dm-core/model.rb
@@ -86,6 +86,12 @@ def self.descendants
     # @api semipublic
     def self.append_inclusions(*inclusions)
       extra_inclusions.concat inclusions
+
+      # Add the inclusion to existing descendants
+      descendants.each do |model|
+        inclusions.each { |inclusion| model.send :include, inclusion }
+      end
+
       true
     end
 
@@ -111,6 +117,12 @@ def self.extra_inclusions
     # @api semipublic
     def self.append_extensions(*extensions)
       extra_extensions.concat extensions
+
+      # Add the extension to existing descendants
+      descendants.each do |model|
+        extensions.each { |extension| model.extend(extension) }
+      end
+
       true
     end
 
diff --git a/spec/semipublic/model_spec.rb b/spec/semipublic/model_spec.rb
new file mode 100644
index 00000000..79c8059d
--- /dev/null
+++ b/spec/semipublic/model_spec.rb
@@ -0,0 +1,96 @@
+require File.expand_path(File.join(File.dirname(__FILE__), '..', 'spec_helper'))
+
+describe DataMapper::Model do
+
+  it { should respond_to(:append_inclusions) }
+
+  describe '.append_inclusions' do
+    module ::Inclusions
+      def new_method
+      end
+    end
+
+    describe 'before the model is defined' do
+      before :all do
+        DataMapper::Model.append_inclusions(Inclusions)
+
+        class ::User
+          include DataMapper::Resource
+          property :id, Serial
+        end
+      end
+
+      it 'should respond to :new_method' do
+        User.new.should respond_to(:new_method)
+      end
+
+      after :all do
+        DataMapper::Model.extra_inclusions.delete(Inclusions)
+      end
+    end
+
+    describe 'after the model is defined' do
+      before :all do
+        class ::User
+          include DataMapper::Resource
+          property :id, Serial
+        end
+        DataMapper::Model.append_inclusions(Inclusions)
+      end
+
+      it 'should respond to :new_method' do
+        User.new.should respond_to(:new_method)
+      end
+
+      after :all do
+        DataMapper::Model.extra_inclusions.delete(Inclusions)
+      end
+    end
+  end
+
+  it { should respond_to(:append_extensions) }
+
+  describe '.append_extensions' do
+    module ::Extensions
+      def new_method
+      end
+    end
+
+    describe 'before the model is defined' do
+      before :all do
+        DataMapper::Model.append_extensions(Extensions)
+
+        class ::User
+          include DataMapper::Resource
+          property :id, Serial
+        end
+      end
+
+      it 'should respond to :new_method' do
+        User.should respond_to(:new_method)
+      end
+
+      after :all do
+        DataMapper::Model.extra_extensions.delete(Extensions)
+      end
+    end
+
+    describe 'after the model is defined' do
+      before :all do
+        class ::User
+          include DataMapper::Resource
+          property :id, Serial
+        end
+        DataMapper::Model.append_extensions(Extensions)
+      end
+
+      it 'should respond to :new_method' do
+        User.should respond_to(:new_method)
+      end
+
+      after :all do
+        DataMapper::Model.extra_extensions.delete(Extensions)
+      end
+    end
+  end
+end