use django checks system

Author: shaleh

oauth2_provider/checks.py

@@ -0,0 +1,29 @@
+from django.apps import apps
+from django.core import checks
+from django.db import router
+
+from .settings import oauth2_settings
+
+
+@checks.register(checks.Tags.database)
+def validate_token_configuration(app_configs, **kwargs):
+    breakpoint()
+    databases = set(
+        router.db_for_write(apps.get_model(model))
+        for model in (
+            oauth2_settings.ACCESS_TOKEN_MODEL,
+            oauth2_settings.ID_TOKEN_MODEL,
+            oauth2_settings.REFRESH_TOKEN_MODEL,
+        )
+    )
+
+    # This is highly unlikely, but let's warn people just in case it does.
+    # If the tokens were allowed to be in different databases this would require all
+    # writes to have a transaction around each database. Instead, let's enforce that
+    # they all live together in one database.
+    # The tokens are not required to live in the default database provided the Django
+    # routers know the correct database for them.
+    if len(databases) > 1:
+        return [checks.Error("The token models are expected to be stored in the same database.")]
+
+    return []