feat(postgrest): add current timestamp sentinel

src/postgrest/src/postgrest/constants.py

@@ -1,6 +1,14 @@
+from datetime import datetime
+from typing import Final, cast
+
 DEFAULT_POSTGREST_CLIENT_HEADERS = {
     "Accept": "application/json",
     "Content-Type": "application/json",
 }
 
 DEFAULT_POSTGREST_CLIENT_TIMEOUT = 120
+
+# PostgreSQL recognizes this string as the current transaction timestamp. The
+# cast lets generated update types accept it for datetime columns while keeping
+# the value sent to PostgREST unchanged.
+NOW: Final[datetime] = cast(datetime, "now()")

src/postgrest/tests/test_constants.py

@@ -0,0 +1,14 @@
+from datetime import datetime
+from typing import TypedDict
+
+from postgrest.constants import NOW
+
+
+class GeneratedUpdate(TypedDict, total=False):
+    updated_at: datetime
+
+
+def test_now_is_compatible_with_generated_datetime_fields() -> None:
+    payload = GeneratedUpdate(updated_at=NOW)
+
+    assert payload == {"updated_at": "now()"}