update code structure and get session on each query

app/database.py

@@ -0,0 +1,14 @@
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker, scoped_session
+from app.config import Config
+
+engine = create_engine(Config.DATABASE_URL, pool_pre_ping=True)
+Session = scoped_session(sessionmaker(bind=engine))
+
+
+def get_db_session():
+    return Session()
+
+
+def close_db_session(exception=None):
+    Session.remove()