diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index fd0caa6..367ce4f 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -7,6 +7,7 @@ ChangeLog
 0.2.1
 -----
 - Fix bug in windows.
+- Enhance PostgreSQL support.
 
 0.2.0
 -----
diff --git a/aerich/ddl/postgres/__init__.py b/aerich/ddl/postgres/__init__.py
index 88b2de2..1bac896 100644
--- a/aerich/ddl/postgres/__init__.py
+++ b/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
 
diff --git a/tests/test_ddl.py b/tests/test_ddl.py
index b07ba1a..999b8e8 100644
--- a/tests/test_ddl.py
+++ b/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:
@@ -88,7 +91,7 @@ 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):
@@ -96,13 +99,13 @@ def test_alter_column_default():
             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():
@@ -110,7 +113,7 @@ def test_alter_column_null():
     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():
@@ -118,13 +121,13 @@ def test_set_comment():
     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():