feat: support commit timestamp option

Add support for columns with commit timestamps:
https://cloud.google.com/spanner/docs/commit-timestamp

Fixes: #695

Author: waltaskew

README.rst

@@ -234,6 +234,22 @@ tables with this feature, make sure to call ``add_is_dependent_on()`` on
 the child table to request SQLAlchemy to create the parent table before
 the child table.
 
+Commit timestamps
+~~~~~~~~~~~~~~~~~~
+
+The dialect offers the ``spanner_allow_commit_timestamp`` option to
+column constructors for creating commit timestamp columns.
+
+.. code:: python
+
+   Table(
+       "table",
+       metadata,
+       Column("last_update_time", DateTime, spanner_allow_commit_timestamp=True),
+   )
+
+`See this documentation page for more details <https://cloud.google.com/spanner/docs/commit-timestamp>`__.
+
 Unique constraints
 ~~~~~~~~~~~~~~~~~~
 

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -578,6 +578,11 @@ def get_column_specification(self, column, **kwargs):
         elif hasattr(column, "computed") and column.computed is not None:
             colspec += " " + self.process(column.computed)
 
+        if column.dialect_options.get("spanner", {}).get(
+            "allow_commit_timestamp", False
+        ):
+            colspec += " OPTIONS (allow_commit_timestamp=true)"
+
         return colspec
 
     def visit_computed_column(self, generated, **kw):

test/mockserver_tests/commit_timestamp_model.py

@@ -0,0 +1,32 @@
+# Copyright 2025 Google LLC All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import datetime
+
+from sqlalchemy.orm import DeclarativeBase
+from sqlalchemy.orm import Mapped
+from sqlalchemy.orm import mapped_column
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+class Singer(Base):
+    __tablename__ = "singers"
+    id: Mapped[str] = mapped_column(primary_key=True)
+    name: Mapped[str]
+    updated_at: Mapped[datetime.datetime] = mapped_column(
+        spanner_allow_commit_timestamp=True
+    )

test/mockserver_tests/test_commit_timestamp.py

@@ -0,0 +1,64 @@
+# Copyright 2025 Google LLC All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from sqlalchemy import create_engine
+from sqlalchemy.testing import eq_, is_instance_of
+from google.cloud.spanner_v1 import (
+    FixedSizePool,
+    ResultSet,
+)
+from test.mockserver_tests.mock_server_test_base import (
+    MockServerTestBase,
+    add_result,
+)
+from google.cloud.spanner_admin_database_v1 import UpdateDatabaseDdlRequest
+
+
+class TestCommitTimestamp(MockServerTestBase):
+    def test_create_table(self):
+        from test.mockserver_tests.commit_timestamp_model import Base
+
+        add_result(
+            """SELECT true
+FROM INFORMATION_SCHEMA.TABLES
+WHERE TABLE_SCHEMA="" AND TABLE_NAME="singers"
+LIMIT 1
+""",
+            ResultSet(),
+        )
+        add_result(
+            """SELECT true
+                FROM INFORMATION_SCHEMA.SEQUENCES
+                WHERE NAME="singer_id"
+                AND SCHEMA=""
+                LIMIT 1""",
+            ResultSet(),
+        )
+        engine = create_engine(
+            "spanner:///projects/p/instances/i/databases/d",
+            connect_args={"client": self.client, "pool": FixedSizePool(size=10)},
+        )
+        Base.metadata.create_all(engine)
+        requests = self.database_admin_service.requests
+        eq_(1, len(requests))
+        is_instance_of(requests[0], UpdateDatabaseDdlRequest)
+        eq_(1, len(requests[0].statements))
+        eq_(
+            "CREATE TABLE singers (\n"
+            "\tid STRING(MAX) NOT NULL, \n"
+            "\tname STRING(MAX) NOT NULL, \n"
+            "\tupdated_at TIMESTAMP NOT NULL OPTIONS (allow_commit_timestamp=true)\n"
+            ") PRIMARY KEY (id)",
+            requests[0].statements[0],
+        )