fix: Correctly Generate DDL for ALTER COLUMN ... SET DEFAULT

Alembic expects `get_column_default_string` to be implemented in order
to use it for  `ALTER TABLE.. ALTER COLUMN .. SET DEFAULT` DDL. In
our case, this means wrapping the default value in parentheses. We
implement `get_column_default_string` and have it add parentheses for
use in both `CREATE TABLE` and `ALTER TABLE` DDL.

Call path for alembic relying on `get_column_default_string` is here:

https://github.com/sqlalchemy/alembic/blob/cd4f404358f101b2b930013c609c074baca61468/alembic/ddl/base.py#L252
https://github.com/sqlalchemy/alembic/blob/cd4f404358f101b2b930013c609c074baca61468/alembic/ddl/base.py#L315

Author: waltaskew

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -572,7 +572,7 @@ def get_column_specification(self, column, **kwargs):
         elif has_identity:
             colspec += " " + self.process(column.identity)
         elif default is not None:
-            colspec += " DEFAULT (" + default + ")"
+            colspec += f" DEFAULT {default}"
         elif hasattr(column, "computed") and column.computed is not None:
             colspec += " " + self.process(column.computed)
 
@@ -583,6 +583,13 @@ def get_column_specification(self, column, **kwargs):
 
         return colspec
 
+    def get_column_default_string(self, column):
+        default = super().get_column_default_string(column)
+        if default is not None:
+            return f"({default})"
+
+        return default
+
     def visit_computed_column(self, generated, **kw):
         """Computed column operator."""
         text = "AS (%s) STORED" % self.sql_compiler.process(

test/mockserver_tests/default_model.py

@@ -0,0 +1,30 @@
+# 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 func
+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(
+        server_default=func.GENERATE_UUID(), primary_key=True
+    )
+    name: Mapped[str]

test/mockserver_tests/test_default.py

@@ -0,0 +1,49 @@
+# 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 TestCreateTableDefault(MockServerTestBase):
+    def test_create_table_with_default(self):
+        from test.mockserver_tests.default_model import Base
+
+        add_result(
+            """SELECT true
+FROM INFORMATION_SCHEMA.TABLES
+WHERE TABLE_SCHEMA="" AND TABLE_NAME="singers"
+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 DEFAULT (GENERATE_UUID()), \n"
+            "\tname STRING(MAX) NOT NULL\n"
+            ") PRIMARY KEY (id)",
+            requests[0].statements[0],
+        )