diff --git a/link/model/__init__.py b/link/model/__init__.py
index 1c734d4..0417beb 100644
--- a/link/model/__init__.py
+++ b/link/model/__init__.py
@@ -1,5 +1,5 @@
 # -*- coding: utf-8 -*-
 
-__version__ = '0.3'
+__version__ = '0.4'
 
 CONF_BASE_PATH = 'link/model'
diff --git a/link/model/feature.py b/link/model/feature.py
index 47f5604..7eeaab5 100644
--- a/link/model/feature.py
+++ b/link/model/feature.py
@@ -34,12 +34,12 @@ def __call__(self, schemaname):
 
 
 class Model(object):
-    DATA_ID = 'id'
+    _DATA_ID = 'id'
 
     def __init__(self, middleware, **kwargs):
         super(Model, self).__init__()
 
-        self.middleware = middleware
+        self._middleware = middleware
 
         for key in kwargs:
             self[key] = kwargs[key]
@@ -93,7 +93,7 @@ def __str__(self):
         return str(d)
 
     def __repr__(self):
-        return 'Model(data_id={0})'.format(self[self.DATA_ID])
+        return 'Model(data_id={0})'.format(self[self._DATA_ID])
 
     def save(self):
         raise NotImplementedError()
diff --git a/requirements.txt b/requirements.txt
index f3fef90..01c5604 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,2 +1,2 @@
 b3j0f.conf==0.3.18
-link.middleware==0.16
+link.middleware==0.19
diff --git a/setup.py b/setup.py
index 539e7d9..1cb1dd2 100644
--- a/setup.py
+++ b/setup.py
@@ -14,7 +14,7 @@
 LICENSE = 'MIT'
 REQUIREMENTS = [
     'b3j0f.conf>=0.3.18',
-    'link.middleware>=0.16'
+    'link.middleware>=0.19'
 ]
 
 CLASSIFIERS = [