chore(django-spanner): add unit test coverage

[ohmayr]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request expands test coverage for the django-google-spanner package, adding unit tests for database creation, features, initialization, compiler edge cases, lookups, operations, and schema editor functionality, while also integrating mockserver tests into the main test run. The review feedback correctly identifies several instances where direct modification of global settings, connection features, or module states could lead to test pollution and flaky test runs, and suggests using mock patching or try-finally blocks to ensure clean test isolation.

[gemini-code-assist]

If any assertion fails during the test, the module django_spanner.features will not be reloaded to its original state, which can cause test pollution and flaky failures in subsequent tests. Wrap the test logic in a try...finally block to guarantee that the module is always reloaded.

Suggested change

	with mock.patch.dict(os.environ, {"RUNNING_SPANNER_BACKEND_TESTS": "1", "SPANNER_EMULATOR_HOST": "localhost:9010"}):
	with mock.patch.object(django_spanner, "USE_EMULATOR", True):
	importlib.reload(django_spanner.features)
	feat = django_spanner.features.DatabaseFeatures(self.db_wrapper)
	self.assertFalse(feat.supports_foreign_keys)
	self.assertFalse(feat.supports_json_field)
	self.assertTrue(any("test_loaddata" in test for test in feat.skip_tests))
	importlib.reload(django_spanner.features)
	try:
	with mock.patch.dict(os.environ, {"RUNNING_SPANNER_BACKEND_TESTS": "1", "SPANNER_EMULATOR_HOST": "localhost:9010"}):
	with mock.patch.object(django_spanner, "USE_EMULATOR", True):
	importlib.reload(django_spanner.features)
	feat = django_spanner.features.DatabaseFeatures(self.db_wrapper)
	self.assertFalse(feat.supports_foreign_keys)
	self.assertFalse(feat.supports_json_field)
	self.assertTrue(any("test_loaddata" in test for test in feat.skip_tests))
	finally:
	importlib.reload(django_spanner.features)

[gemini-code-assist]

Modifying compiler.connection.features.supports_slicing_ordering_in_compound directly can pollute other tests if the connection is shared. Use mock.patch.object to safely patch the feature attribute and ensure it is restored after the test.

Suggested change

	compiler = SQLCompiler(qs4.query, self.connection, "default")
	compiler.connection.features.supports_slicing_ordering_in_compound = True
	compiler.query.high_mark = 10
	sql, params = compiler.get_combinator_sql("union", False)
	self.assertTrue(len(sql) > 0)
	compiler = SQLCompiler(qs4.query, self.connection, "default")
	with mock.patch.object(compiler.connection.features, "supports_slicing_ordering_in_compound", True):
	compiler.query.high_mark = 10
	sql, params = compiler.get_combinator_sql("union", False)
	self.assertTrue(len(sql) > 0)

[gemini-code-assist]

Modifying self.db_wrapper.features.skip_tests directly can pollute other tests that share the same connection wrapper. Use mock.patch.object to safely patch skip_tests so that it is automatically restored after the test block.

    def test_mark_skips(self):
        with mock.patch("django.conf.settings.INSTALLED_APPS", ["django.contrib.contenttypes"]):
            with mock.patch.object(self.db_wrapper.features, "skip_tests", ("django.contrib.contenttypes.models.ContentType",)):
                self.creation.mark_skips()

[gemini-code-assist]

Modifying settings.USE_TZ directly inside the loop can leave it in an incorrect state if any assertion fails. Wrap the loop in a try...finally block to guarantee that settings.USE_TZ is always restored to True.

Suggested change

	for use_tz in [True, False]:
	settings.USE_TZ = use_tz
	tz = "IST" if use_tz else "UTC"
	self.assertIn(tz, ops.datetime_extract_sql("dayofweek", "f", None, "IST")[0])
	self.assertIn(tz, ops.datetime_cast_time_sql("f", None, "IST")[0])
	settings.USE_TZ = True
	try:
	for use_tz in [True, False]:
	settings.USE_TZ = use_tz
	tz = "IST" if use_tz else "UTC"
	self.assertIn(tz, ops.datetime_extract_sql("dayofweek", "f", None, "IST")[0])
	self.assertIn(tz, ops.datetime_cast_time_sql("f", None, "IST")[0])
	finally:
	settings.USE_TZ = True