INTPYTHON-772 Fix transaction.atomic() crash if connection isn't initialized

[ddrondo]

I ran into a problem during a transaction when I call the POST create method in the view and try to create documents through the serializer:

class CreateTariff(TariffsSerializer):
    @transaction.atomic(using="mongodb")
    def create(self, validated_data):
        ...

This error occurs spontaneously and always at the first call to the endpoints API.

File: C\test\venv\lib\site-packages\rest_framework\views.py in dispatch (line 506)
response = handler(request, *args, **kwargs)

File: C:\test\venv\lib\site-packages\rest_framework\mixins.py in create (line 19)
self.perform_create(serializer)

File: C:\test\venv\lib\site-packages\rest_framework\mixins.py in perform_create (line 24)
serializer.save()

File: C:\test\venv\lib\site-packages\rest_framework\serializers.py in save (line 208)
self.instance = self.create(validated_data)

...

File: C:\test\venv\lib\site-packages\django_mongodb_backend\transaction.py in __enter__ (line 29)
connection.start_transaction_mongo()

File: C:\test\venv\lib\site-packages\django\utils\asyncio.py in inner (line 26)
return func(*args, **kwargs)

File: C:\test\venv\lib\site-packages\django_mongodb_backend\base.py in start_transaction_mongo (line 275)
self.session = self.connection.start_session()
Error: 'NoneType' object has no attribute 'start_session'

As I understand it, it occurs due to the fact that the connection lazily creates a connection (connection.connection) during the entry into the transaction.

[timgraham]

Thank you. transaction.atomic() is modeled off of django.transaction.atomic() so I don't think we want the logic added there. I'll amend this and add a test.

I don't like having to add ensure_connection() in various places (this also came up in the showencryptedfields management command of #329), but the alternative of a __getattr__() hack [5febcd3 removed it] isn't nice either.

[aclark4life]

LGTM, thanks @ddrondo and @timgraham