add tests for DatabaseFeatures.supports_transactions

Author: timgraham

django_mongodb_backend/features.py

@@ -106,28 +106,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         "test_utils.tests.CaptureOnCommitCallbacksTests.test_with_rolled_back_savepoint",
     }
 
-    @cached_property
-    def supports_transactions(self):
-        """Confirm support for transactions."""
-        is_replica_set = False
-        is_sharded_cluster = False
-        with self.connection.cursor():
-            client = self.connection.connection
-            hello_response = client.admin.command("hello")
-            server_status = client.admin.command("serverStatus")
-            if "setName" in hello_response:
-                is_replica_set = True
-            if "msg" in hello_response and hello_response["msg"] == "isdbgrid":
-                is_sharded_cluster = True
-            if (
-                "storageEngine" in server_status
-                and server_status["storageEngine"].get("name") == "wiredTiger"
-            ):
-                is_wired_tiger = True
-        if (is_replica_set or is_sharded_cluster) and is_wired_tiger:
-            return True
-        return False
-
     @cached_property
     def django_test_expected_failures(self):
         expected_failures = super().django_test_expected_failures
@@ -615,3 +593,19 @@ def supports_atlas_search(self):
     def supports_select_union(self):
         # Stage not supported inside of a multi-document transaction: $unionWith
         return not self.supports_transactions
+
+    @cached_property
+    def supports_transactions(self):
+        """
+        Transactions are enabled if the MongoDB configuration supports it:
+        MongoDB must be configured as a replica set or sharded cluster, and
+        the store engine must be WiredTiger.
+        """
+        client = self.connection.connection.admin
+        hello_response = client.command("hello")
+        is_replica_set = "setName" in hello_response
+        is_sharded_cluster = hello_response.get("msg") == "isdbgrid"
+        if is_replica_set or is_sharded_cluster:
+            engine = client.command("serverStatus").get("storageEngine", {})
+            return engine.get("name") == "wiredTiger"
+        return False

tests/backend_/test_features.py

@@ -0,0 +1,76 @@
+from unittest.mock import patch
+
+from django.db import connection
+from django.test import TestCase
+
+
+class SupportsTransactionsTests(TestCase):
+    def setUp(self):
+        # Clear the cached property.
+        del connection.features.supports_transactions
+
+    def tearDown(self):
+        del connection.features.supports_transactions
+
+    def test_replica_set(self):
+        """A replica set supports transactions."""
+
+        def mocked_command(command):
+            if command == "hello":
+                return {"setName": "foo"}
+            if command == "serverStatus":
+                return {"storageEngine": {"name": "wiredTiger"}}
+            raise Exception("Unexpected command")
+
+        with patch("pymongo.synchronous.database.Database.command", wraps=mocked_command):
+            self.assertIs(connection.features.supports_transactions, True)
+
+    def test_replica_set_other_storage_engine(self):
+        """No support on a non-wiredTiger replica set."""
+
+        def mocked_command(command):
+            if command == "hello":
+                return {"setName": "foo"}
+            if command == "serverStatus":
+                return {"storageEngine": {"name": "other"}}
+            raise Exception("Unexpected command")
+
+        with patch("pymongo.synchronous.database.Database.command", wraps=mocked_command):
+            self.assertIs(connection.features.supports_transactions, False)
+
+    def test_sharded_cluster(self):
+        """A sharded cluster with wiredTiger storage engine supports them."""
+
+        def mocked_command(command):
+            if command == "hello":
+                return {"msg": "isdbgrid"}
+            if command == "serverStatus":
+                return {"storageEngine": {"name": "wiredTiger"}}
+            raise Exception("Unexpected command")
+
+        with patch("pymongo.synchronous.database.Database.command", wraps=mocked_command):
+            self.assertIs(connection.features.supports_transactions, True)
+
+    def test_sharded_cluster_other_storage_engine(self):
+        """No support on a non-wiredTiger shared cluster."""
+
+        def mocked_command(command):
+            if command == "hello":
+                return {"msg": "isdbgrid"}
+            if command == "serverStatus":
+                return {"storageEngine": {"name": "other"}}
+            raise Exception("Unexpected command")
+
+        with patch("pymongo.synchronous.database.Database.command", wraps=mocked_command):
+            self.assertIs(connection.features.supports_transactions, False)
+
+    def test_no_support(self):
+        """No support on a non-replica set, non-sharded cluster."""
+
+        def mocked_command(command):
+            if command == "hello":
+                return {}
+            raise Exception("Unexpected command")
+
+        with patch("pymongo.synchronous.database.Database.command", wraps=mocked_command):
+            self.assertIs(connection.features.supports_transactions, False)