[MINOR] add opt-in multi-database support for db fixtures

docs/database.rst

@@ -66,22 +66,34 @@ select using an argument to the ``django_db`` mark::
 Tests requiring multiple databases
 ----------------------------------
 
-Currently ``pytest-django`` does not specifically support Django's
-multi-database support.
+You can configure ``pytest-django`` to create transactions (or cleanup) databases
+other than ``default`` by adding a special key to your database test configuration
+in your django settings.
+
+for example::
+
+    DATABASES = {
+        "default": {
+            # ... normal default settings
+        },
+        "secondary": {
+            # ... engine settings and so forth
+            "TEST": {
+                "PYTEST_DJANGO_ALLOW_TRANSACTIONS": True
+            }
+        }
+    }
+
+
+With that database configuration in your test environment, tests that depend on the
+``db`` or ``transactional_db`` fixtures can make changes in the "secondary" database
+and have it cleaned up after each test.
 
-You can however use normal :class:`~django.test.TestCase` instances to use its
+You could also use normal :class:`~django.test.TestCase` instances to use its
 :ref:`django:topics-testing-advanced-multidb` support.
 In particular, if your database is configured for replication, be sure to read
 about :ref:`django:topics-testing-primaryreplica`.
 
-If you have any ideas about the best API to support multiple databases
-directly in ``pytest-django`` please get in touch, we are interested
-in eventually supporting this but unsure about simply following
-Django's approach.
-
-See `pull request 431 <https://github.com/pytest-dev/pytest-django/pull/431>`_
-for an idea/discussion to approach this.
-
 ``--reuse-db`` - reuse the testing database between test runs
 --------------------------------------------------------------
 Using ``--reuse-db`` will create the test database in the same way as

pytest_django/fixtures.py

@@ -124,7 +124,7 @@ def teardown_database():
 
 
 def _django_db_fixture_helper(
-    request, django_db_blocker, transactional=False, reset_sequences=False
+    request, django_db_blocker, settings, transactional=False, reset_sequences=False,
 ):
     if is_django_unittest(request):
         return
@@ -148,11 +148,37 @@ class ResetSequenceTestCase(django_case):
     else:
         from django.test import TestCase as django_case
 
+    # specify attributes on test cases for multi-database support
+    # https://docs.djangoproject.com/en/3.1/topics/testing/tools/#multi-database-support
+    transactional_databases = _transactional_databases(settings)
+    if transactional_databases:
+
+        class MultiDatabaseTransactionTestCase(django_case):
+            databases = transactional_databases
+
+        django_case = MultiDatabaseTransactionTestCase
+        request.config.django_transactional_databases = transactional_databases
+
     test_case = django_case(methodName="__init__")
     test_case._pre_setup()
     request.addfinalizer(test_case._post_teardown)
 
 
+def _transactional_databases(settings):
+    """
+    Get database labels from settings in which pytest-django should start a transaction.
+    """
+    transactional_databases = {"default"}
+    for label, config in settings.DATABASES.items():
+        if label == "default":
+            continue
+
+        if config.get("TEST", {}).get("PYTEST_DJANGO_ALLOW_TRANSACTIONS"):
+            transactional_databases.add(label)
+
+    return transactional_databases
+
+
 def _disable_native_migrations():
     from django.conf import settings
     from django.core.management.commands import migrate
@@ -196,7 +222,7 @@ def _set_suffix_to_test_databases(suffix):
 
 
 @pytest.fixture(scope="function")
-def db(request, django_db_setup, django_db_blocker):
+def db(request, django_db_setup, django_db_blocker, settings):
     """Require a django test database.
 
     This database will be setup with the default fixtures and will have
@@ -218,11 +244,11 @@ def db(request, django_db_setup, django_db_blocker):
     ):
         request.getfixturevalue("transactional_db")
     else:
-        _django_db_fixture_helper(request, django_db_blocker, transactional=False)
+        _django_db_fixture_helper(request, django_db_blocker, settings, transactional=False)
 
 
 @pytest.fixture(scope="function")
-def transactional_db(request, django_db_setup, django_db_blocker):
+def transactional_db(request, django_db_setup, django_db_blocker, settings):
     """Require a django test database with transaction support.
 
     This will re-initialise the django database for each test and is
@@ -237,11 +263,11 @@ def transactional_db(request, django_db_setup, django_db_blocker):
     """
     if "django_db_reset_sequences" in request.fixturenames:
         request.getfixturevalue("django_db_reset_sequences")
-    _django_db_fixture_helper(request, django_db_blocker, transactional=True)
+    _django_db_fixture_helper(request, django_db_blocker, settings, transactional=True)
 
 
 @pytest.fixture(scope="function")
-def django_db_reset_sequences(request, django_db_setup, django_db_blocker):
+def django_db_reset_sequences(request, django_db_setup, django_db_blocker, settings):
     """Require a transactional test database with sequence reset support.
 
     This behaves like the ``transactional_db`` fixture, with the addition
@@ -254,7 +280,7 @@ def django_db_reset_sequences(request, django_db_setup, django_db_blocker):
     ``transactional_db``, ``django_db_reset_sequences``.
     """
     _django_db_fixture_helper(
-        request, django_db_blocker, transactional=True, reset_sequences=True
+        request, django_db_blocker, settings, transactional=True, reset_sequences=True
     )
 
 

pytest_django_test/settings_sqlite.py

@@ -4,5 +4,5 @@
     "default": {
         "ENGINE": "django.db.backends.sqlite3",
         "NAME": "/should_not_be_accessed",
-    }
+    },
 }

pytest_django_test/settings_sqlite_file.py

@@ -13,5 +13,12 @@
         "ENGINE": "django.db.backends.sqlite3",
         "NAME": "/pytest_django_should_never_get_accessed",
         "TEST": {"NAME": _filename},
+    },
+    "secondary": {
+        "ENGINE": "django.db.backends.sqlite3",
+        "NAME": "/should_not_be_accessed_two",
+        "TEST": {
+            "PYTEST_DJANGO_ALLOW_TRANSACTIONS": True
+        }
     }
 }

tests/test_database.py

@@ -1,5 +1,5 @@
 import pytest
-from django.db import connection
+from django.db import connection, connections
 from django.test.testcases import connections_support_transactions
 
 from pytest_django_test.app.models import Item
@@ -138,6 +138,26 @@ def test_fin(self, fin):
         # Check finalizer has db access (teardown will fail if not)
         pass
 
+    def test_db_sets_transactions_in_all_configured_connections(self, settings, db):
+        assert all(
+            conn.in_atomic_block
+            for conn in connections.all() if conn.features.supports_transactions
+        )
+
+    def test_multi_database_true_if_settings_permit_db(self, request, settings, transactional_db):
+        transactional_databases = set()
+        for label, config in settings.DATABASES.items():
+            if config.get("TEST", {}).get("PYTEST_DJANGO_ALLOW_TRANSACTIONS"):
+                transactional_databases.add(label)
+
+        if not transactional_databases:
+            pytest.skip(
+                "settings.DATABASES is not configured to create transactions"
+                "in databases outside of default"
+            )
+
+        assert {"default", "secondary"} == request.config.django_transactional_databases
+
 
 class TestDatabaseFixturesAllOrder:
     @pytest.fixture