fix test error

CHANGELOG.rst

@@ -7,6 +7,7 @@ ChangeLog
 0.2.1
 -----
 - Fix bug in windows.
+- Enhance PostgreSQL support.
 
 0.2.0
 -----

aerich/ddl/postgres/__init__.py

@@ -1,3 +1,6 @@
+from typing import Type
+
+from tortoise import Model
 from tortoise.backends.asyncpg.schema_generator import AsyncpgSchemaGenerator
 from tortoise.fields import Field
 

tests/test_ddl.py

@@ -73,7 +73,10 @@ def test_modify_column():
 
     ret = Migrate.ddl.modify_column(User, User._meta.fields_map.get("is_active"))
     if isinstance(Migrate.ddl, MysqlDDL):
-        assert ret == "ALTER TABLE `user` MODIFY COLUMN `is_active` BOOL NOT NULL"
+        assert (
+            ret
+            == "ALTER TABLE `user` MODIFY COLUMN `is_active` BOOL NOT NULL  COMMENT 'Is Active' DEFAULT 1"
+        )
     elif isinstance(Migrate.ddl, PostgresDDL):
         assert ret == 'ALTER TABLE "user" ALTER COLUMN "is_active" TYPE BOOL'
     else:
@@ -88,43 +91,43 @@ def test_alter_column_default():
     if isinstance(Migrate.ddl, PostgresDDL):
         assert ret == 'ALTER TABLE "category" ALTER COLUMN "name" DROP DEFAULT'
     else:
-        assert ret == None
+        assert ret is None
 
     ret = Migrate.ddl.alter_column_default(Category, Category._meta.fields_map.get("created_at"))
     if isinstance(Migrate.ddl, PostgresDDL):
         assert (
             ret == 'ALTER TABLE "category" ALTER COLUMN "created_at" SET DEFAULT CURRENT_TIMESTAMP'
         )
     else:
-        assert ret == None
+        assert ret is None
 
     ret = Migrate.ddl.alter_column_default(User, User._meta.fields_map.get("avatar"))
     if isinstance(Migrate.ddl, PostgresDDL):
         assert ret == 'ALTER TABLE "user" ALTER COLUMN "avatar" SET DEFAULT \'\''
     else:
-        assert ret == None
+        assert ret is None
 
 
 def test_alter_column_null():
     ret = Migrate.ddl.alter_column_null(Category, Category._meta.fields_map.get("name"))
     if isinstance(Migrate.ddl, PostgresDDL):
         assert ret == 'ALTER TABLE "category" ALTER COLUMN "name" SET NOT NULL'
     else:
-        assert ret == None
+        assert ret is None
 
 
 def test_set_comment():
     ret = Migrate.ddl.set_comment(Category, Category._meta.fields_map.get("name"))
     if isinstance(Migrate.ddl, PostgresDDL):
         assert ret == 'COMMENT ON COLUMN "category"."name" IS NULL'
     else:
-        assert ret == None
+        assert ret is None
 
     ret = Migrate.ddl.set_comment(Category, Category._meta.fields_map.get("user"))
     if isinstance(Migrate.ddl, PostgresDDL):
         assert ret == 'COMMENT ON COLUMN "category"."user" IS \'User\''
     else:
-        assert ret == None
+        assert ret is None
 
 
 def test_drop_column():