ENG-3288 Add onupdate to PrivacyPreferences.updated_at column (#7851)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: erosselli

changelog/7851-updated-at-onupdate.yaml

@@ -0,0 +1,4 @@
+type: Fixed
+description: Fixed updated_at column on privacy_preferences to auto-populate on record updates
+pr: 7851
+labels: []

src/fides/api/models/v3/privacy_preferences.py

@@ -12,6 +12,7 @@
 from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.ext.declarative import declared_attr
 from sqlalchemy.orm import Session
+from sqlalchemy.sql import func
 
 from fides.api.cryptography.cryptographic_util import hash_value_with_salt
 from fides.api.cryptography.identity_salt import get_identity_salt
@@ -76,14 +77,23 @@ def __tablename__(self) -> str:
         Boolean, nullable=False, server_default=text("false"), primary_key=True
     )
 
-    # Override base class timestamp columns to match migration
+    # Override base class timestamp columns:
+    # - created_at: indexed for query performance (base class doesn't index it)
+    # - updated_at: nullable with no server_default (unlike base class which defaults
+    #   to now()). New records start with updated_at=NULL; it is only set when a
+    #   record is later modified (this only happens when is_latest is flipped
+    #   from True to False).
     created_at = Column(
         DateTime(timezone=True),
         nullable=False,
         server_default=text("now()"),
         index=True,
     )
-    updated_at = Column(DateTime(timezone=True), nullable=True)
+    updated_at = Column(
+        DateTime(timezone=True),
+        nullable=True,
+        onupdate=func.now(),
+    )
 
     @classmethod
     def hash_value(