fixed two issues:

add_article: called flush and has been changed to flush()
NewsArticleSchema: published_date UnixTimestamp(dump_only=True)

news/app.py

@@ -66,7 +66,7 @@ def add_article(session: Session, request_data: http.RequestData, router: Router
     news_article = NewsArticle(**news_article_data)
 
     session.add(news_article)
-    session.flush
+    session.flush()
 
     headers = {"Location": router.reverse_url('get_source', {"id": news_article.id})}
 

news/schema.py

@@ -59,7 +59,7 @@ class NewsArticleSchema(Schema):
     url = fields.URL(required=True, error_messages={'required': 'A NewsArticle must include a URL'})
     title = fields.Str()
     authors = fields.List(fields.Str())
-    published_date = UnixTimestamp()
+    published_date = UnixTimestamp(dump_only=True)
     news_blob = fields.Str(required=True, error_messages={'required': 'NewsArticle must include news content'})
 
     # TODO add support for Tags