feat: Support Server-Side Checks for Enums (googleapis#694)

Spanner uses protos for enums. Creating a column like
Column("an_enum", Enum("A", "B", "C")) will result in a String
column. Setting supports_native_enum to False allows SQLAlchemy
to generate check constraints to enforce the enum values if the
create_constraint=True flag is passed to the Enum constructor.

Fixes: googleapis#686

Author: waltaskew

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -807,10 +807,15 @@ class SpannerDialect(DefaultDialect):
     supports_sequences = True
     sequences_optional = False
     supports_identity_columns = True
-    supports_native_enum = True
     supports_native_boolean = True
     supports_native_decimal = True
     supports_statement_cache = True
+    # Spanner uses protos for enums. Creating a column like
+    # Column("an_enum", Enum("A", "B", "C")) will result in a String
+    # column. Setting supports_native_enum to False allows SQLAlchemy
+    # to generate check constraints to enforce the enum values if the
+    # create_constraint=True flag is passed to the Enum constructor.
+    supports_native_enum = False
 
     postfetch_lastrowid = False
     insert_returning = True

test/mockserver_tests/test_basics.py

@@ -27,6 +27,7 @@
     func,
     text,
     BigInteger,
+    Enum,
 )
 from sqlalchemy.orm import Session, DeclarativeBase, Mapped, mapped_column
 from sqlalchemy.testing import eq_, is_instance_of
@@ -120,6 +121,7 @@ def test_create_table(self):
             metadata,
             Column("user_id", Integer, primary_key=True),
             Column("user_name", String(16), nullable=False),
+            Column("status", Enum("a", "b", "cee", create_constraint=True)),
         )
         metadata.create_all(engine)
         requests = self.database_admin_service.requests
@@ -130,7 +132,9 @@ def test_create_table(self):
             "CREATE TABLE users (\n"
             "\tuser_id INT64 NOT NULL "
             "GENERATED BY DEFAULT AS IDENTITY (BIT_REVERSED_POSITIVE), \n"
-            "\tuser_name STRING(16) NOT NULL\n"
+            "\tuser_name STRING(16) NOT NULL, \n"
+            "\tstatus STRING(3), \n"
+            "\tCHECK (status IN ('a', 'b', 'cee'))\n"
             ") PRIMARY KEY (user_id)",
             requests[0].statements[0],
         )