Incompatibilities with pymongo 4.0

[bagerard]

There is a breaking change in pymongo 4.0 that was released very recently

It fails whenever we save an instance right now

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/usr/local/lib/python3.8/site-packages/mongoengine/document.py:407: in save
    self.ensure_indexes()
/usr/local/lib/python3.8/site-packages/mongoengine/document.py:897: in ensure_indexes
    collection = cls._get_collection()
/usr/local/lib/python3.8/site-packages/mongoengine/document.py:220: in _get_collection
    cls.ensure_indexes()
/usr/local/lib/python3.8/site-packages/mongoengine/document.py:900: in ensure_indexes
    if not collection.is_mongos and collection.read_preference > 1:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
self = Collection(Database(MongoClient(host=['mongo:27017'], document_class=dict, tz_aware=False, connect=True, read_preference=Primary()), 'cpreview_test'), 'test_document.is_mongos')
    def __bool__(self):
>       raise NotImplementedError("Collection objects do not implement truth "
                                  "value testing or bool(). Please compare "
                                  "with None instead: collection is not None")
E       NotImplementedError: Collection objects do not implement truth value testing or bool(). Please compare with None instead: collection is not None
/usr/local/lib/python3.8/site-packages/pymongo/collection.py:300: NotImplementedError

[bagerard]

There are actually much more things that breaks with pymongo 4.0, the list of breaking changes is quite long https://pymongo.readthedocs.io/en/stable/changelog.html#breaking-changes-in-4-0

[victorherbemontagne]

Same problem here, we found a very bad workaround right now. (calling the function twice.... ?)

Does anybody found a better solution right now than rolling back to the previous version ?

Thanks ! :)

[bagerard]

This might work but it will break elsewhere for sure, I'd strongly advise you not to upgrade to pymongo 4.0 with mongoengine until this ticket gets resolved

[ShaneHarvey]

PyMongo maintainer here. This error actually indicates some dead-code in mongoengine. This if statement will never evaluate to True:

/usr/local/lib/python3.8/site-packages/mongoengine/document.py:900: in ensure_indexes
    if not collection.is_mongos and collection.read_preference > 1:

In PyMongo <4.0 collection.is_mongos is always True because it's a new Collection object. If "collection" is referencing a coll named "db.coll", then collection.is_mongos is a coll named "db.coll.is_mongos":

>>> collection = client.db.coll
>>> collection.full_name
'db.coll'
>>> collection.is_mongos.full_name
'db.coll.is_mongos'

Issues like this are exactly why we made the "Collection objects do not implement truth value testing or bool()" breaking change in 4.0.

collection.read_preference > 1 is also problematic since you can't compare a Collection or a read preference object.

Looking at the original issue for these lines of code I believe this check can simply be removed. PyMongo ignores the read preference for coll.create_index and always sends the command to the primary. coll.create_index is a write so read preference does not apply.

I'll follow the rest of this thread and try to help out as I can.

[bagerard]

Hi @ShaneHarvey , thanks a lot for helping on this.

You are absolutely right and probably the initial intent was to use [MongoClient.is_mongos] (https://pymongo.readthedocs.io/en/stable/api/pymongo/mongo_client.html#pymongo.mongo_client.MongoClient.is_mongos) instead of Collection.is_mongos but anyway, that non-working code was commited 7 years ago and is anyway not useful so we can easily throw that away.

I'll trigger a pipeline with our test suite running against pymongo 4.0, we can then use this as a starting point

[bagerard]

@ShaneHarvey , we still have a bunch of errors. Things that were removed but the pymongo 4.0 changelog doesn't provide information on what's the alternative (if any).

• c.admin.authenticate("admin", "password")
• pymongo.GEOHAYSTACK
• collection.map_reduce & collection.inline_map_reduce

Can you advise on how to migrate these things to the new pymongo?

[bagerard]

Woops I ve just seen the migration guide link in the changelog , I ll get through that first 😁

[victorherbemontagne]

Hi @bagerard you are right, plenty stuff crash right now, do you need help on the migration ?

[MuhammadAfzaal113]

I'm still getting the same error, can you guys please mention what exactly needs to change?

[MuhammadAfzaal113]

I just install django-mongoengine and it solved the problem, thanks a lot.

[bagerard]

@victorherbemontagne Any help is welcome, feel free to branch out the current state of this PR #2600 and help on any of the error, then open another PR. There are a lot of failures but not so much distinct errors

[bagerard]

@MuhammadAfzaal113 I don't see how django-mongoengine would help, it's only providing a thin wrapper to glue mongoengine and django, it's not going to magically solves the incompatibility but maybe it prevents recent pymongo version from being installed