diff --git a/docs/userguide.rst b/docs/userguide.rst
index c196fd16..56bf9c97 100644
--- a/docs/userguide.rst
+++ b/docs/userguide.rst
@@ -213,8 +213,6 @@ saved::
 
     >>> page = Page(title="Test Page")
     >>> page.id
-    ...
-    AttributeError('_id')
     >>> page.save()
     >>> page.id
     ObjectId('123456789abcdef000000000')
diff --git a/mongoengine/base.py b/mongoengine/base.py
index 2e6fe52a..cd6d8bab 100644
--- a/mongoengine/base.py
+++ b/mongoengine/base.py
@@ -28,12 +28,10 @@ class BaseField(object):
         # Get value from document instance if available, if not use default
         value = instance._data.get(self.name)
         if value is None:
-            if self.default is not None:
-                value = self.default
-                if callable(value):
-                    value = value()
-            else:
-                raise AttributeError(self.name)
+            value = self.default
+            # Allow callable default values
+            if callable(value):
+                value = value()
         return value
 
     def __set__(self, instance, value):
@@ -227,8 +225,8 @@ class BaseDocument(object):
 
     def __contains__(self, name):
         try:
-            getattr(self, name)
-            return True
+            val = getattr(self, name)
+            return val is not None
         except AttributeError:
             return False
 
diff --git a/mongoengine/fields.py b/mongoengine/fields.py
index e97aadb2..1163d51a 100644
--- a/mongoengine/fields.py
+++ b/mongoengine/fields.py
@@ -181,9 +181,8 @@ class ReferenceField(BaseField):
         if isinstance(document, (str, unicode, pymongo.objectid.ObjectId)):
             id_ = document
         else:
-            try:
-                id_ = document.id
-            except:
+            id_ = document.id
+            if id_ is None:
                 raise ValidationError('You can only reference documents once '
                                       'they have been saved to the database')
 
diff --git a/tests/document.py b/tests/document.py
index 40dec4d6..83866207 100644
--- a/tests/document.py
+++ b/tests/document.py
@@ -228,7 +228,7 @@ class DocumentTest(unittest.TestCase):
         person_obj = collection.find_one({'name': 'Test User'})
         self.assertEqual(person_obj['name'], 'Test User')
         self.assertEqual(person_obj['age'], 30)
-        self.assertEqual(person_obj['_id'], person.id)
+        self.assertEqual(str(person_obj['_id']), person.id)
 
     def test_delete(self):
         """Ensure that document may be deleted using the delete method.
diff --git a/tests/fields.py b/tests/fields.py
index 49c3f70f..9bad1ea1 100644
--- a/tests/fields.py
+++ b/tests/fields.py
@@ -46,7 +46,7 @@ class FieldTest(unittest.TestCase):
             name = StringField()
         
         person = Person(name='Test User')
-        self.assertRaises(AttributeError, getattr, person, 'id')
+        self.assertEqual(person.id, None)
         self.assertRaises(ValidationError, person.__setattr__, 'id', 47)
         self.assertRaises(ValidationError, person.__setattr__, 'id', 'abc')
         person.id = '497ce96f395f2f052a494fd4'
@@ -173,8 +173,8 @@ class FieldTest(unittest.TestCase):
         post.author = PowerUser(name='Test User', power=47)
 
     def test_reference_validation(self):
-        """Ensure that invalid embedded documents cannot be assigned to
-        embedded document fields.
+        """Ensure that invalid docment objects cannot be assigned to reference
+        fields.
         """
         class User(Document):
             name = StringField()
@@ -187,10 +187,12 @@ class FieldTest(unittest.TestCase):
 
         user = User(name='Test User')
 
+        # Ensure that the referenced object must have been saved
         post1 = BlogPost(content='Chips and gravy taste good.')
         post1.author = user
         self.assertRaises(ValidationError, post1.save)
 
+        # Check that an invalid object type cannot be used
         post2 = BlogPost(content='Chips and chilli taste good.')
         self.assertRaises(ValidationError, post1.__setattr__, 'author', post2)