fix relation serialize issue when relation is empty

tests/server/app/schema.py

@@ -172,6 +172,7 @@ async def get_top_articles(cls, *pks):
     articles: List[ArticleBase] = orm.Field('contents__article')
     # test multi+fk
 
+    follower_names: List[str] = orm.Field('followers.username')
     # @property
     # def total_views(self) -> int:
     #     return sum([article.views for article in self.articles])

tests/test_1_orm/test_schema_query.py

@@ -18,6 +18,7 @@ def test_serialize_users(self, service):
         assert len(res) == 2
         assert res[0]["username"] == "alice"
         assert res[0]["followers_num"] == 2
+        assert set(res[0].follower_names) == {'bob', 'jack'}
         assert res[0]["followings_num"] == 1
         assert set(res[0]["liked_slugs"]) == {"about-tech", "some-news", "big-shot"}
         assert res[0]["@views"] == 103
@@ -41,6 +42,7 @@ def test_serialize_users(self, service):
         sup = UserSchema.init(5)
         assert len(sup.articles) == 0
         assert sup.articles_num == 0
+        assert sup.follower_names == []
 
     def test_scope_and_excludes(self):
         from app.schema import UserSchema, UserQuery
@@ -87,8 +89,8 @@ def test_init_articles(self, service):
         assert content.article.id == 1
 
     def test_related_qs(self):
-        from app.schema import UserBase, ArticleSchema, UserQuery
-        from app.models import Article, Follow, User
+        from app.schema import UserBase, ArticleSchema
+        from app.models import Article, User
         from typing import List, Optional
         from utilmeta.core import orm
         from django.db import models
@@ -155,6 +157,7 @@ async def test_async_init_users(self):
         assert user.sum_views == 103
         assert user.top_articles[0].author_tag["name"] == "alice"
         assert user.top_articles[0].views == 103
+        assert set(user.follower_names) == {'bob', 'jack'}
 
         # --------------
         bob = await UserSchema.ainit(
@@ -170,6 +173,7 @@ async def test_async_init_users(self):
         sup = UserSchema.init(5)
         assert len(sup.articles) == 0
         assert sup.articles_num == 0
+        assert sup.follower_names == []
 
     @pytest.mark.asyncio
     async def test_async_init_users_with_sync_query(self):

utilmeta/core/orm/fields/field.py

@@ -236,6 +236,7 @@ def setup(self, options: utype.Options):
                 # if not self.model_field.is_exp:
                 # expression need to be isolated, otherwise multiple many included query will blow the query
                 self.isolated = True
+                self.related_single = False
 
             elif not self.model_field.is_concrete:
                 self.isolated = True