fix: exclude commit_timestamp col from THEN RETURN

Author: olavloite

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -425,7 +425,14 @@ def returning_clause(self, stmt, returning_cols, **kw):
             self._label_select_column(
                 None, c, True, False, {"spanner_is_returning": True}
             )
-            for c in expression._select_iterables(returning_cols)
+            for c in expression._select_iterables(
+                filter(
+                    lambda col: not col.dialect_options.get("spanner", {}).get(
+                        "exclude_from_returning", False
+                    ),
+                    returning_cols,
+                )
+            )
         ]
 
         return "THEN RETURN " + ", ".join(columns)

test/system/test_basics.py

@@ -316,6 +316,8 @@ class TimestampUser(Base):
             updated_at: Mapped[datetime.datetime] = mapped_column(
                 spanner_allow_commit_timestamp=True,
                 default=text("PENDING_COMMIT_TIMESTAMP()"),
+                # Make sure that this column is never part of a THEN RETURN clause.
+                spanner_exclude_from_returning=True,
             )
 
         @event.listens_for(TimestampUser, "before_update")