diff --git a/lib/disposable/twin/composition.rb b/lib/disposable/twin/composition.rb
index e170f3e..fbb41bc 100644
--- a/lib/disposable/twin/composition.rb
+++ b/lib/disposable/twin/composition.rb
@@ -28,6 +28,22 @@ module ClassMethods
         def expose_class
           @expose_class ||= Class.new(Disposable::Composition).from(representer_class)
         end
+
+        def on(model_key, &block)
+          builder = OnBuilder.new(model_key, self)
+          builder.instance_eval(&block)
+        end
+
+        class OnBuilder
+          def initialize(model_key, definition)
+            @model_key = model_key
+            @definition = definition
+          end
+
+          def property(name, options={}, &block)
+            @definition.property(name, options.merge(on: @model_key), &block)
+          end
+        end
       end
 
       def self.included(base)
@@ -54,4 +70,4 @@ def save_model
       end
     end
   end
-end
\ No newline at end of file
+end
diff --git a/test/twin/composition_test.rb b/test/twin/composition_test.rb
index 2e80bc7..7b8b3dc 100644
--- a/test/twin/composition_test.rb
+++ b/test/twin/composition_test.rb
@@ -11,8 +11,11 @@ class Request < Disposable::Twin
     property :song_id,    on: :song, from: :id
 
     property :name,       on: :requester
-    property :id,         on: :requester
     property :captcha,    readable: false, writeable: false, on: :requester # TODO: allow both, virtual with and without :on.
+
+    on :requester do
+      property :id
+    end
   end
 
   module Model
@@ -81,4 +84,4 @@ module Model
     song.instance_eval { def save; false; end }
     request.save.must_equal false
   end
-end
\ No newline at end of file
+end