add auto setup for Django app

src/email_relay/apps.py

@@ -1,8 +1,37 @@
 from __future__ import annotations
 
 from django.apps import AppConfig
+from django.conf import global_settings
+from django.conf import settings
+
+try:
+    from environs import Env
+except ImportError:
+    Env = None
 
 
 class EmailRelayConfig(AppConfig):
     name = "email_relay"
     default_auto_field = "django.db.models.BigAutoField"
+
+    def ready(self) -> None:
+        from email_relay.conf import app_settings
+
+        if not app_settings.ENABLE_AUTO_SETUP:
+            return
+
+        if (
+            settings.EMAIL_BACKEND is None
+            or settings.EMAIL_BACKEND == global_settings.EMAIL_BACKEND
+        ):
+            settings.EMAIL_BACKEND = "email_relay.backend.EmailRelayBackend"
+
+        if app_settings.DATABASE_ALIAS not in settings.DATABASES and Env is not None:
+            env = Env()
+
+            settings.DATABASES[app_settings.DATABASE_ALIAS] = env.dj_db_url(
+                "EMAIL_RELAY_DATABASE_URL", default="sqlite://:memory:"
+            )
+
+        if "email_relay.db.EmailDatabaseRouter" not in settings.DATABASE_ROUTERS:
+            settings.DATABASE_ROUTERS.append("email_relay.db.EmailDatabaseRouter")

src/email_relay/conf.py

@@ -17,6 +17,7 @@ class AppSettings:
     EMAIL_MAX_RETRIES: int | None = None
     EMPTY_QUEUE_SLEEP: int = 30
     EMAIL_THROTTLE: int = 0
+    ENABLE_AUTO_SETUP: bool = False
     MESSAGES_BATCH_SIZE: int | None = None
     MESSAGES_RETENTION_SECONDS: int | None = None
     RELAY_HEALTHCHECK_METHOD: str = "GET"

tests/test_auto_setup.py

@@ -0,0 +1,50 @@
+from __future__ import annotations
+
+import pytest
+from django.apps import apps
+from django.conf import settings
+from django.test.utils import override_settings
+
+from email_relay.conf import EMAIL_RELAY_DATABASE_ALIAS
+
+
+@pytest.fixture(autouse=True, scope="module")
+def auto_setup_setting():
+    with override_settings(
+        DJANGO_EMAIL_RELAY={
+            "ENABLE_AUTO_SETUP": True,
+        },
+    ):
+        yield
+
+
+@pytest.fixture
+def app_config():
+    return apps.get_app_config("email_relay")
+
+
+@override_settings(EMAIL_BACKEND=None)
+def test_auto_setup_email_backend(app_config):
+    app_config.ready()
+
+    assert settings.EMAIL_BACKEND == "email_relay.backend.EmailRelayBackend"
+
+
+@override_settings(
+    DATABASES={
+        "default": {
+            "ENGINE": "django.db.backends.sqlite3",
+            "NAME": ":memory:",
+        },
+    },
+)
+def test_auto_setup_database(app_config):
+    app_config.ready()
+
+    assert EMAIL_RELAY_DATABASE_ALIAS in settings.DATABASES
+
+
+def test_auto_setup_database_router(app_config):
+    app_config.ready()
+
+    assert "email_relay.db.EmailDatabaseRouter" in settings.DATABASE_ROUTERS