Why does PyMongo give Unsupported projection option: $substr when listing collections? - mongodb

Question

I have the following code:
client = MongoClient(uri)
db = client['my_db']
print(db.collection_names())
#print(db.list_collection_names())
and I get the error
File "C:\Users\gwerner004\eclipse-workspace\MongoTestRasa\FirstTest.py", line 17, in connect
print(db.collection_names())
File "C:\Users\gwerner004\AppData\Local\Programs\Python\Python36\lib\site-packages\pymongo\database.py", line 715, in collection_names
nameOnly=True, **kws)]
File "C:\Users\gwerner004\AppData\Local\Programs\Python\Python36\lib\site-packages\pymongo\database.py", line 677, in list_collections
**kwargs)
File "C:\Users\gwerner004\AppData\Local\Programs\Python\Python36\lib\site-packages\pymongo\database.py", line 651, in _list_collections
cursor = self._command(sock_info, cmd, slave_okay)["cursor"]
File "C:\Users\gwerner004\AppData\Local\Programs\Python\Python36\lib\site-packages\pymongo\database.py", line 514, in _command
client=self.__client)
File "C:\Users\gwerner004\AppData\Local\Programs\Python\Python36\lib\site-packages\pymongo\pool.py", line 579, in command
unacknowledged=unacknowledged)
File "C:\Users\gwerner004\AppData\Local\Programs\Python\Python36\lib\site-packages\pymongo\network.py", line 150, in command
parse_write_concern_error=parse_write_concern_error)
File "C:\Users\gwerner004\AppData\Local\Programs\Python\Python36\lib\site-packages\pymongo\helpers.py", line 155, in _check_command_response
raise OperationFailure(msg % errmsg, code, response)
pymongo.errors.OperationFailure: Unsupported projection option: $substr
Why do I get a failure for such a basic operation? I am running on Windows 10 and using Python 3.6.7. My PyMongo is 3.7.2

Answer 1

The $substr operator works on all currently supported MongoDB versions (2.6-4.4):
> db.foo.aggregate([{$project:{"name": {"$substr": ["$name", 2, -1]}}}])
{ "_id" : ObjectId("5fc032e56bc5b2e2216cdd08"), "name" : "llo" }
Most likely you are using either an ancient MongoDB installation or, per one of the comments, an imitation database like CosmosDB that does not behave as MongoDB itself would (which are also not supported by official MongoDB drivers).