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: #732

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
@@ -1853,11 +1854,14 @@ 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,
     )
 
 
@@ -1866,9 +1870,28 @@ 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
+):
+    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/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())",
+        )