transaction.atomic() crashes if connection isn't initialized

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      Reported on GitHub PR 412: 

      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.

            Assignee:
            Tim Graham
            Reporter:
            Tim Graham
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              Created:
              Updated:
              Resolved: