US07-05: Deliver Backup and Operational Recovery (#92)

photo_pipeline/db.py

@@ -41,12 +41,41 @@ def create_session_factory(engine: Engine) -> sessionmaker:
     return sessionmaker(bind=engine, expire_on_commit=False, future=True)
 
 
-def run_migrations(url: str) -> None:
-    """Upgrade the database at ``url`` to the latest revision."""
-    from alembic import command
+def _alembic_config(url: str):
     from alembic.config import Config as AlembicConfig
 
     cfg = AlembicConfig(str(_REPO_ROOT / "alembic.ini"))
     cfg.set_main_option("script_location", str(_REPO_ROOT / "migrations"))
     cfg.set_main_option("sqlalchemy.url", url)
-    command.upgrade(cfg, "head")
+    return cfg
+
+
+def run_migrations(url: str) -> None:
+    """Upgrade the database at ``url`` to the latest revision."""
+    from alembic import command
+
+    command.upgrade(_alembic_config(url), "head")
+
+
+def head_revision() -> str | None:
+    """The revision this code expects. ``None`` if the scripts cannot be read."""
+    from alembic.script import ScriptDirectory
+
+    try:
+        return ScriptDirectory.from_config(_alembic_config("sqlite://")).get_current_head()
+    except Exception:
+        return None
+
+
+def current_revision(url: str) -> str | None:
+    """The revision a database is actually at, or ``None`` for an unstamped one."""
+    engine = create_db_engine(url)
+    try:
+        with engine.connect() as connection:
+            from alembic.runtime.migration import MigrationContext
+
+            return MigrationContext.configure(connection).get_current_revision()
+    except Exception:
+        return None
+    finally:
+        engine.dispose()