Can't connect to MongoDB after upgrading?

Question

I'm building a website using Flask in which I use MongoDB with the MongoEngine ORM. To go for a fresh start again I now upgraded all apt and pip packages on my ubuntu 14.04 development machine. Unfortunately this broke my connection to MongoDB:

Traceback (most recent call last):
  File "./run.py", line 4, in <module>
    from app import app, socketio
  File "/home/kr65/beta/app/__init__.py", line 21, in <module>
    mongoDb = MongoEngine(app)
  File "/usr/local/lib/python2.7/dist-packages/flask_mongoengine/__init__.py", line 33, in __init__
    self.init_app(app)
  File "/usr/local/lib/python2.7/dist-packages/flask_mongoengine/__init__.py", line 66, in init_app
    self.connection = mongoengine.connect(**conn_settings)
  File "/usr/local/lib/python2.7/dist-packages/mongoengine/connection.py", line 164, in connect
    return get_connection(alias)
  File "/usr/local/lib/python2.7/dist-packages/mongoengine/connection.py", line 126, in get_connection
    raise ConnectionError("Cannot connect to database %s :\n%s" % (alias, e))
mongoengine.connection.ConnectionError: Cannot connect to database default :
False is not a read preference.

I checked if MongoDB is up:

$ sudo service mongodb status
mongodb start/running, process 781

and if I could get into the interactive command line:

$ mongo
MongoDB shell version: 2.4.9
connecting to: test
Welcome to the MongoDB shell.
For interactive help, type "help".
For more comprehensive documentation, see
    http://docs.mongodb.org/
Questions? Try the support group
    http://groups.google.com/group/mongodb-user
Server has startup warnings:
Tue Apr 14 09:14:10.267 [initandlisten]
Tue Apr 14 09:14:10.267 [initandlisten] ** WARNING: You are running in OpenVZ. This is known to be broken!!!
Tue Apr 14 09:14:10.267 [initandlisten]
>

I didn't change anything to the code or passwords or anything like that. I did a reboot and restarted mongoDB, but nothing works. My settings are like this:

MONGODB_SETTINGS = {
    'db': 'mydatabasename'
}

and I instantiate the connection like this (which worked before):

app = Flask(__name__)
app.config.from_object('config')
mongoDb = MongoEngine(app)

Since I didn't really change anything, I'm kind of unsure where to search for a solution. Does anybody have any tips how I could solve this?

[EDIT] With the tip of @lapinkoira my MongoDB now starts up correctly, but I now get the error below while querying. Any ideas how to solve this one?

File "/home/kr65/beta/app/views/webviews.py", line 476, in getDoc
    userDoc = UserDocument.objects(id=docId).first()
  File "/usr/local/lib/python2.7/dist-packages/mongoengine/queryset/base.py", line 309, in first
    result = queryset[0]
  File "/usr/local/lib/python2.7/dist-packages/mongoengine/queryset/base.py", line 160, in __getitem__
    return queryset._document._from_son(queryset._cursor[key],
  File "/usr/local/lib/python2.7/dist-packages/mongoengine/queryset/base.py", line 1410, in _cursor
    **self._cursor_args)
  File "/usr/local/lib/python2.7/dist-packages/pymongo/collection.py", line 924, in find
    return Cursor(self, *args, **kwargs)
TypeError: __init__() got an unexpected keyword argument 'snapshot'

Answer 1

looks like you have pymongo 3.0 installed.

Mongoengine is not yet compatible with it.

You can try fix this by

pip uninstall pymongo
pip install pymongo==2.8