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

django_mongodb_backend/base.py

@@ -272,6 +272,7 @@ def get_database_version(self):
     @async_unsafe
     def start_transaction_mongo(self):
         if self.session is None:
+            self.ensure_connection()
             self.session = self.connection.start_session()
             with debug_transaction(self, "session.start_transaction()"):
                 self.session.start_transaction()

docs/releases/5.2.x.rst

@@ -23,6 +23,8 @@ Bug fixes
   :attr:`~django.db.models.Field.db_column`.
 - Corrected the search index type of ``EmbeddedModelField`` and
   ``PolymorphicEmbeddedModelField`` from ``embeddedDocuments`` to ``document``.
+- Fixed ``transaction.atomic()`` crash if the database connection isn't
+  initialized.
 
 Deprecated features
 -------------------

tests/transactions_/tests.py

@@ -1,4 +1,4 @@
-from django.db import DatabaseError
+from django.db import DatabaseError, connection
 from django.test import TransactionTestCase, skipIfDBFeature, skipUnlessDBFeature
 
 from django_mongodb_backend import transaction
@@ -140,6 +140,12 @@ def __call__(self):
 
         transaction.atomic(Callable())  # Must not raise an exception
 
+    def test_initializes_connection(self):
+        """transaction.atomic() opens the connection if needed."""
+        connection.close_pool()
+        with transaction.atomic():
+            pass
+
 
 @skipIfDBFeature("_supports_transactions")
 class AtomicNotSupportedTests(TransactionTestCase):