add negative test

Author: Danil Tolmachev

tests/test_glom_writing.py

@@ -1,3 +1,5 @@
+import pytest
+
 import dictdatabase as DDB
 
 data = {
@@ -15,3 +17,11 @@ def test_glom_writing():
         purchase["status"] = "cancelled"
         session.write()
     assert DDB.at("users", key="users.Ben.status").read() == "cancelled"
+
+
+def test_glom_writing_sub_key_not_exists():
+    DDB.at("users").create(data, force_overwrite=True)
+    with pytest.raises(KeyError):
+        with DDB.at("users", key="users.SUBKEY").session() as (session, purchase):
+            purchase["status"] = "cancelled"
+            session.write()