Demo: resolve user names from database state, kanta instances at top with stacked decorators

demo/main.py

@@ -17,24 +17,12 @@ from types import ModuleType
 import msgspec
 
 from kanta import Kanta
+from kanta.callbacks import DictPost, DictPre
 from kanta.logging import _ACTION, _ACTOR, _RESET, _SESSION, _TARGET
 from kanta.logging import configure_logging
 
 DB = Path(__file__).with_name("demo.kantadb")
 
-# Fake directory: user id -> display name, resolved by the logfmt callbacks.
-USERS = {"u1": "Alice", "u2": "Bob", "u3": "Carol"}
-
-_now = datetime(2026, 8, 6, tzinfo=UTC)
-
-
-def fake_now() -> datetime:
-    """Deterministic clock: starts at midnight, +1h on every read."""
-    global _now
-    ts = _now
-    _now += timedelta(hours=1)
-    return ts
-
 
 class DataV1(msgspec.Struct):
     """Original schema (version 0)."""
@@ -59,19 +47,41 @@ def migrate_v1(d: dict) -> None:
 migrations = ModuleType("demo_migrations")
 migrations.migrate_v1 = migrate_v1
 
+# Phase 1 instance: default logging, original schema.
+kanta_v0 = Kanta(DB, DataV1())
+# Phase 2 instance: migrations and a custom log header.
+kanta_v1 = Kanta(DB, Data(), migrations=migrations)
 
-def resolve_actor(value: str) -> str | None:
+_now = datetime(2026, 8, 6, tzinfo=UTC)
-    """Resolve the transaction user id to a display name."""
-    return USERS.get(value)
 
 
-def resolve_user_key(value: str, path: str) -> str | None:
+@kanta_v0.clock
-    """Resolve user ids in diff paths to display names."""
+@kanta_v1.clock
-    if path.startswith("users."):
+def fake_now() -> datetime:
-        return USERS.get(value)
+    """Deterministic clock: starts at midnight, +1h on every read."""
+    global _now
+    ts = _now
+    _now += timedelta(hours=1)
+    return ts
+
+
+@kanta_v0.logfmt
+@kanta_v1.logfmt
+def resolve_user(
+    value: str, path: str, previous: DictPre, current: DictPost
+) -> str | None:
+    """Resolve user ids to names from the database state itself."""
+    if path != "$user" and not path.startswith("users."):
+        return None
+    # Post-change state first, then pre-change (deleted users are only there).
+    for state in (current, previous):
+        name = state.get("users", {}).get(value, {}).get("name")
+        if name:
+            return name
     return None
 
 
+@kanta_v1.logheader
 def header(action: str, user: str | None, extra: dict | None) -> str:
     """Aligned rich header: actor, session id, action, target."""
     actor = f"{_ACTOR}{user or '-':<8}{_RESET}"
@@ -80,25 +90,12 @@ def header(action: str, user: str | None, extra: dict | None) -> str:
     return f"{actor} {session}  {_ACTION}{action}{_RESET}  {target}"
 
 
+@kanta_v0.bootstrap
 def seed(data: DataV1) -> None:
-    """Bootstrap: create the initial admin user."""
+    """Create the initial admin user."""
     data.users["u1"] = {"name": "Alice", "role": "admin"}
 
 
-# Phase 1 instance: default logging, original schema.
-kanta_v0 = Kanta(DB, DataV1())
-# Phase 2 instance: migrations and a custom log header.
-kanta_v1 = Kanta(DB, Data(), migrations=migrations)
-
-for k in (kanta_v0, kanta_v1):
-    k.clock(fake_now)
-    k.logfmt(resolve_user_key)
-    k.logfmt(resolve_actor, path="$user")
-
-kanta_v0.bootstrap(seed)
-kanta_v1.logheader(header)
-
-
 def section(title: str) -> None:
     print(f"\n# {title}", flush=True)
 
@@ -157,7 +154,7 @@ async def main() -> None:
 
     with kanta_v1.transaction(
         action="update",
-        user="u2",
+        user="u3",
         extra={"session_id": 7, "target": "settings (demo)"},
     ) as data:
         data.settings["lang"] = "en"