fix: Respect existing server default in alter column DDL (googleapis#733)

* 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

* fix: Respect existing server default in alter column DDL

Include `DEFAULT (default_value)` stanzas in `ALTER COLUMN` DDL to
avoid removing `DEFAULT` values when changing a column's type or
nullability.

Fixes: googleapis#732

* chore: ignore booleans

---------

Co-authored-by: Knut Olav Løite <koloite@gmail.com>

Author: waltaskew

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -20,6 +20,7 @@
     ColumnType,
     alter_column,
     alter_table,
+    format_server_default,
     format_type,
 )
 from google.api_core.client_options import ClientOptions
@@ -579,7 +580,7 @@ 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)
 
@@ -590,6 +591,13 @@ 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(
@@ -1860,11 +1868,14 @@ def do_execute_no_params(self, cursor, statement, context=None):
 def visit_column_nullable(
     element: "ColumnNullable", compiler: "SpannerDDLCompiler", **kw
 ) -> str:
-    return "%s %s %s %s" % (
-        alter_table(compiler, element.table_name, element.schema),
-        alter_column(compiler, element.column_name),
-        format_type(compiler, element.existing_type),
-        "" if element.nullable else "NOT NULL",
+    return _format_alter_column(
+        compiler,
+        element.table_name,
+        element.schema,
+        element.column_name,
+        element.existing_type,
+        element.nullable,
+        element.existing_server_default,
     )
 
 
@@ -1873,9 +1884,34 @@ def visit_column_nullable(
 def visit_column_type(
     element: "ColumnType", compiler: "SpannerDDLCompiler", **kw
 ) -> str:
-    return "%s %s %s %s" % (
-        alter_table(compiler, element.table_name, element.schema),
-        alter_column(compiler, element.column_name),
-        "%s" % format_type(compiler, element.type_),
-        "" if element.existing_nullable else "NOT NULL",
+    return _format_alter_column(
+        compiler,
+        element.table_name,
+        element.schema,
+        element.column_name,
+        element.type_,
+        element.existing_nullable,
+        element.existing_server_default,
+    )
+
+
+def _format_alter_column(
+    compiler, table_name, schema, column_name, type_, nullable, server_default
+):
+    # Older versions of SQLAlchemy pass in a boolean to indicate whether there
+    # is an existing DEFAULT constraint, instead of the actual DEFAULT constraint
+    # expression. In those cases, we do not want to explicitly include the DEFAULT
+    # constraint in the expression that is generated here.
+    if isinstance(server_default, bool):
+        server_default = None
+    return "%s %s %s%s%s" % (
+        alter_table(compiler, table_name, schema),
+        alter_column(compiler, column_name),
+        format_type(compiler, type_),
+        "" if nullable else " NOT NULL",
+        (
+            ""
+            if server_default is None
+            else f" DEFAULT {format_server_default(compiler, server_default)}"
+        ),
     )

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],
+        )

test/unit/__init__.py

@@ -0,0 +1,97 @@
+# Copyright 2025 Google LLC
+#
+# 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 alembic.ddl import base as ddl_base
+from google.cloud.sqlalchemy_spanner import sqlalchemy_spanner
+from sqlalchemy import String, TextClause
+from sqlalchemy.testing import eq_
+from sqlalchemy.testing.plugin.plugin_base import fixtures
+
+
+class TestAlembicTest(fixtures.TestBase):
+    def test_visit_column_nullable_with_not_null_column(self):
+        ddl = sqlalchemy_spanner.visit_column_nullable(
+            ddl_base.ColumnNullable(
+                name="tbl", column_name="col", nullable=False, existing_type=String(256)
+            ),
+            sqlalchemy_spanner.SpannerDDLCompiler(
+                sqlalchemy_spanner.SpannerDialect(), None
+            ),
+        )
+        eq_(ddl, "ALTER TABLE tbl ALTER COLUMN col STRING(256) NOT NULL")
+
+    def test_visit_column_nullable_with_nullable_column(self):
+        ddl = sqlalchemy_spanner.visit_column_nullable(
+            ddl_base.ColumnNullable(
+                name="tbl", column_name="col", nullable=True, existing_type=String(256)
+            ),
+            sqlalchemy_spanner.SpannerDDLCompiler(
+                sqlalchemy_spanner.SpannerDialect(), None
+            ),
+        )
+        eq_(ddl, "ALTER TABLE tbl ALTER COLUMN col STRING(256)")
+
+    def test_visit_column_nullable_with_default(self):
+        ddl = sqlalchemy_spanner.visit_column_nullable(
+            ddl_base.ColumnNullable(
+                name="tbl",
+                column_name="col",
+                nullable=False,
+                existing_type=String(256),
+                existing_server_default=TextClause("GENERATE_UUID()"),
+            ),
+            sqlalchemy_spanner.SpannerDDLCompiler(
+                sqlalchemy_spanner.SpannerDialect(), None
+            ),
+        )
+        eq_(
+            ddl,
+            "ALTER TABLE tbl "
+            "ALTER COLUMN col "
+            "STRING(256) NOT NULL DEFAULT (GENERATE_UUID())",
+        )
+
+    def test_visit_column_type(self):
+        ddl = sqlalchemy_spanner.visit_column_type(
+            ddl_base.ColumnType(
+                name="tbl",
+                column_name="col",
+                type_=String(256),
+                existing_nullable=True,
+            ),
+            sqlalchemy_spanner.SpannerDDLCompiler(
+                sqlalchemy_spanner.SpannerDialect(), None
+            ),
+        )
+        eq_(ddl, "ALTER TABLE tbl ALTER COLUMN col STRING(256)")
+
+    def test_visit_column_type_with_default(self):
+        ddl = sqlalchemy_spanner.visit_column_type(
+            ddl_base.ColumnType(
+                name="tbl",
+                column_name="col",
+                type_=String(256),
+                existing_nullable=False,
+                existing_server_default=TextClause("GENERATE_UUID()"),
+            ),
+            sqlalchemy_spanner.SpannerDDLCompiler(
+                sqlalchemy_spanner.SpannerDialect(), None
+            ),
+        )
+        eq_(
+            ddl,
+            "ALTER TABLE tbl "
+            "ALTER COLUMN col "
+            "STRING(256) NOT NULL DEFAULT (GENERATE_UUID())",
+        )