fix for reassign Table Keys #171 (#174)

* fix for reassign Table Keys #171

* change comment, because line was to long for linter (< 88 characters)

* add unittest to check "insert > delete > insert" for a key in table

faust/stores/memory.py

@@ -31,7 +31,9 @@ def apply_changelog_batch(
             self._create_batch_iterator(to_delete.add, to_key, to_value, batch)
         )
         for key in to_delete:
-            delete_key(key, None)
+            # If the key was assigned a value again, it will not be deleted.
+            if not self.data[key]:
+                delete_key(key, None)
 
     def _create_batch_iterator(
         self,
@@ -45,7 +47,6 @@ def _create_batch_iterator(
             # to delete keys in the table we set the raw value to None
             if event.message.value is None:
                 mark_as_delete(key)
-                continue
             yield key, to_value(event.value)
 
     def persisted_offset(self, tp: TP) -> Optional[int]:

tests/unit/stores/test_memory.py

@@ -33,6 +33,15 @@ def test_apply_changelog_batch__deletes_key_for_None_value(self, *, store):
 
         assert to_key() not in store.data
 
+    def test_apply_changelog_batch__deletes_key_and_reassign_it(self, *, store):
+        self.test_apply_changelog_batch__deletes_key_for_None_value(store=store)
+
+        events = [self.mock_event(value=value) for value in ("v1", None, "v2")]
+        to_key, to_value = self.mock_to_key_value(events[0])
+
+        store.apply_changelog_batch(events, to_key=to_key, to_value=to_value)
+        assert to_key() in store.data
+
     def mock_event_to_key_value(self, key=b"key", value=b"value"):
         event = self.mock_event(key=key, value=value)
         to_key, to_value = self.mock_to_key_value(event)