Created ObjectIdField, removed object_id field parameter

tests/document.py

@@ -31,6 +31,7 @@ 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)
         # Test iteration over fields
         fields = list(Person())
         self.assertTrue('name' in fields and 'age' in fields)
@@ -67,7 +68,8 @@ class DocumentTest(unittest.TestCase):
         person['name'] = 'Another User'
         self.assertEquals(person['name'], 'Another User')
 
-        self.assertEquals(len(person), 2)
+        # Length = length(assigned fields + _id)
+        self.assertEquals(len(person), 3)
 
         self.assertTrue('age' in person)
         person.age = None
@@ -81,6 +83,7 @@ class DocumentTest(unittest.TestCase):
             content = StringField()
         
         self.assertTrue('content' in Comment._fields)
+        self.assertFalse('_id' in Comment._fields)
         self.assertFalse(hasattr(Comment, '_meta'))
 
     def test_save(self):
@@ -95,6 +98,18 @@ class DocumentTest(unittest.TestCase):
         self.assertEqual(person_obj['name'], 'Test User')
         self.assertEqual(person_obj['age'], 30)
 
+    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')
+        person.save()
+        # Ensure that the object is in the database with the correct _id
+        collection = self.db[self.Person._meta['collection']]
+        person_obj = collection.find_one({'name': 'Test User'})
+        self.assertEqual(str(person_obj['_id']), '497ce96f395f2f052a494fd4')
+
     def test_save_embedded_document(self):
         """Ensure that a document with an embedded document field may be 
         saved in the database.