Changed the inheritance model to remove types

The inheritance model has changed, we no longer need to store an array of
`types` with the model we can just use the classname in `_cls`.

See the upgrade docs for information on how to upgrade
MongoEngine/mongoengine#148

tests/migration/turn_off_inheritance.py

@@ -0,0 +1,62 @@
+# -*- coding: utf-8 -*-
+import unittest
+
+from mongoengine import Document, connect
+from mongoengine.connection import get_db
+from mongoengine.fields import StringField
+
+__all__ = ('TurnOffInheritanceTest', )
+
+
+class TurnOffInheritanceTest(unittest.TestCase):
+
+    def setUp(self):
+        connect(db='mongoenginetest')
+        self.db = get_db()
+
+    def tearDown(self):
+        for collection in self.db.collection_names():
+            if 'system.' in collection:
+                continue
+            self.db.drop_collection(collection)
+
+    def test_how_to_turn_off_inheritance(self):
+        """Demonstrates migrating from allow_inheritance = True to False.
+        """
+
+        # 1. Old declaration of the class
+
+        class Animal(Document):
+            name = StringField()
+            meta = {
+                'allow_inheritance': True,
+                'indexes': ['name']
+            }
+
+        # 2. Turn off inheritance
+        class Animal(Document):
+            name = StringField()
+            meta = {
+                'allow_inheritance': False,
+                'indexes': ['name']
+            }
+
+        # 3. Remove _types and _cls
+        collection = Animal._get_collection()
+        collection.update({}, {"$unset": {"_types": 1, "_cls": 1}}, multi=True)
+
+        # 3. Confirm extra data is removed
+        count = collection.find({"$or": [{'_types': {"$exists": True}},
+                                         {'_cls': {"$exists": True}}]}).count()
+        assert count == 0
+
+        # 4. Remove indexes
+        info = collection.index_information()
+        indexes_to_drop = [key for key, value in info.iteritems()
+                           if '_types' in dict(value['key'])
+                              or '_cls' in dict(value['key'])]
+        for index in indexes_to_drop:
+            collection.drop_index(index)
+
+        # 5. Recreate indexes
+        Animal.objects._ensure_indexes()