Renamed Document._id to id (still _id in DB)

Although MongoDB uses _id, underscore prefixed attributes imply
private access in Python and are sometimes may not be accessed
(e.g. in the Django template language), but id should be public.

tests/document.py

@@ -41,7 +41,7 @@ class DocumentTest(unittest.TestCase):
         self.assertEqual(Person._fields['name'], name_field)
         self.assertEqual(Person._fields['age'], age_field)
         self.assertFalse('non_field' in Person._fields)
-        self.assertTrue('_id' in Person._fields)
+        self.assertTrue('id' in Person._fields)
         # Test iteration over fields
         fields = list(Person())
         self.assertTrue('name' in fields and 'age' in fields)
@@ -145,7 +145,7 @@ class DocumentTest(unittest.TestCase):
         person['name'] = 'Another User'
         self.assertEquals(person['name'], 'Another User')
 
-        # Length = length(assigned fields + _id)
+        # Length = length(assigned fields + id)
         self.assertEquals(len(person), 3)
 
         self.assertTrue('age' in person)
@@ -160,7 +160,7 @@ class DocumentTest(unittest.TestCase):
             content = StringField()
         
         self.assertTrue('content' in Comment._fields)
-        self.assertFalse('_id' in Comment._fields)
+        self.assertFalse('id' in Comment._fields)
         self.assertFalse(hasattr(Comment, '_meta'))
 
     def test_save(self):
@@ -174,14 +174,14 @@ 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(person_obj['_id'], person.id)
 
     def test_save_custom_id(self):
         """Ensure that a document may be saved with a custom _id.
         """
         # Create person object and save it to the database
         person = self.Person(name='Test User', age=30, 
-                             _id='497ce96f395f2f052a494fd4')
+                             id='497ce96f395f2f052a494fd4')
         person.save()
         # Ensure that the object is in the database with the correct _id
         collection = self.db[self.Person._meta['collection']]
@@ -268,7 +268,7 @@ class DocumentTest(unittest.TestCase):
         post_obj.author.age = 25
         post_obj.author.save()
 
-        author = self.Person.objects(name='Test User').first()
+        author = list(self.Person.objects(name='Test User'))[-1]
         self.assertEqual(author.age, 25)
 
         BlogPost.drop_collection()