We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
Rename modifier missing from update
Not sure if this is intentional or not but it would be useful to have the `$rename` operator (or "modifier" for the update method for QuerySet and Document) available.

I'm currently working around it with `exec_js`, like so:

``` python
Document.objects.exec_js("""
function() {
  db[collection].update({}, {$rename: {foo: 'bar'}});
}""")
```


--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `mongoengine/base/common.py`
Content:
```
1 from mongoengine.errors import NotRegistered
2 
3 __all__ = ('UPDATE_OPERATORS', 'get_document', '_document_registry')
4 
5 
6 UPDATE_OPERATORS = set(['set', 'unset', 'inc', 'dec', 'pop', 'push',
7                         'push_all', 'pull', 'pull_all', 'add_to_set',
8                         'set_on_insert', 'min', 'max'])
9 
10 
11 _document_registry = {}
12 
13 
14 def get_document(name):
15     """Get a document class by name."""
16     doc = _document_registry.get(name, None)
17     if not doc:
18         # Possible old style name
19         single_end = name.split('.')[-1]
20         compound_end = '.%s' % single_end
21         possible_match = [k for k in _document_registry.keys()
22                           if k.endswith(compound_end) or k == single_end]
23         if len(possible_match) == 1:
24             doc = _document_registry.get(possible_match.pop(), None)
25     if not doc:
26         raise NotRegistered("""
27             `%s` has not been registered in the document registry.
28             Importing the document class automatically registers it, has it
29             been imported?
30         """.strip() % name)
31     return doc
32
```
--- END FILES ---

Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.

Here is an example:

```diff
diff --git a/examples/server_async.py b/examples/server_async.py
--- a/examples/server_async.py
+++ b/examples/server_async.py
@@ -313,4 +313,4 @@
 
 
 if __name__ == "__main__":
-    asyncio.run(run_async_server("."), debug=True)
+    asyncio.run(run_async_server(), debug=True)
diff --git a/examples/server_sync.py b/examples/server_sync.py
--- a/examples/server_sync.py
+++ b/examples/server_sync.py
@@ -313,5 +313,5 @@
 
 
 if __name__ == "__main__":
-    server = run_sync_server(".")
+    server = run_sync_server()
     server.shutdown()

```

diff --git a/mongoengine/base/common.py b/mongoengine/base/common.py
--- a/mongoengine/base/common.py
+++ b/mongoengine/base/common.py
@@ -5,7 +5,7 @@
 
 UPDATE_OPERATORS = set(['set', 'unset', 'inc', 'dec', 'pop', 'push',
                         'push_all', 'pull', 'pull_all', 'add_to_set',
-                        'set_on_insert', 'min', 'max'])
+                        'set_on_insert', 'min', 'max', 'rename'])
 
 
 _document_registry = {}

{"golden_diff": "diff --git a/mongoengine/base/common.py b/mongoengine/base/common.py\n--- a/mongoengine/base/common.py\n+++ b/mongoengine/base/common.py\n@@ -5,7 +5,7 @@\n \n UPDATE_OPERATORS = set(['set', 'unset', 'inc', 'dec', 'pop', 'push',\n                         'push_all', 'pull', 'pull_all', 'add_to_set',\n-                        'set_on_insert', 'min', 'max'])\n+                        'set_on_insert', 'min', 'max', 'rename'])\n \n \n _document_registry = {}\n", "issue": "Rename modifier missing from update\nNot sure if this is intentional or not but it would be useful to have the `$rename` operator (or \"modifier\" for the update method for QuerySet and Document) available.\n\nI'm currently working around it with `exec_js`, like so:\n\n``` python\nDocument.objects.exec_js(\"\"\"\nfunction() {\n  db[collection].update({}, {$rename: {foo: 'bar'}});\n}\"\"\")\n```\n\n", "before_files": [{"content": "from mongoengine.errors import NotRegistered\n\n__all__ = ('UPDATE_OPERATORS', 'get_document', '_document_registry')\n\n\nUPDATE_OPERATORS = set(['set', 'unset', 'inc', 'dec', 'pop', 'push',\n                        'push_all', 'pull', 'pull_all', 'add_to_set',\n                        'set_on_insert', 'min', 'max'])\n\n\n_document_registry = {}\n\n\ndef get_document(name):\n    \"\"\"Get a document class by name.\"\"\"\n    doc = _document_registry.get(name, None)\n    if not doc:\n        # Possible old style name\n        single_end = name.split('.')[-1]\n        compound_end = '.%s' % single_end\n        possible_match = [k for k in _document_registry.keys()\n                          if k.endswith(compound_end) or k == single_end]\n        if len(possible_match) == 1:\n            doc = _document_registry.get(possible_match.pop(), None)\n    if not doc:\n        raise NotRegistered(\"\"\"\n            `%s` has not been registered in the document registry.\n            Importing the document class automatically registers it, has it\n            been imported?\n        \"\"\".strip() % name)\n    return doc\n", "path": "mongoengine/base/common.py"}], "after_files": [{"content": "from mongoengine.errors import NotRegistered\n\n__all__ = ('UPDATE_OPERATORS', 'get_document', '_document_registry')\n\n\nUPDATE_OPERATORS = set(['set', 'unset', 'inc', 'dec', 'pop', 'push',\n                        'push_all', 'pull', 'pull_all', 'add_to_set',\n                        'set_on_insert', 'min', 'max', 'rename'])\n\n\n_document_registry = {}\n\n\ndef get_document(name):\n    \"\"\"Get a document class by name.\"\"\"\n    doc = _document_registry.get(name, None)\n    if not doc:\n        # Possible old style name\n        single_end = name.split('.')[-1]\n        compound_end = '.%s' % single_end\n        possible_match = [k for k in _document_registry.keys()\n                          if k.endswith(compound_end) or k == single_end]\n        if len(possible_match) == 1:\n            doc = _document_registry.get(possible_match.pop(), None)\n    if not doc:\n        raise NotRegistered(\"\"\"\n            `%s` has not been registered in the document registry.\n            Importing the document class automatically registers it, has it\n            been imported?\n        \"\"\".strip() % name)\n    return doc\n", "path": "mongoengine/base/common.py"}]}