fix: empty Entities for optional LocalStructuredProperty fields (#370)

* fix: empty Entities for optional LocalStructuredProperty fields

Fixes #369

Author: davidsansome

packages/google-cloud-ndb/google/cloud/ndb/model.py

@@ -4360,9 +4360,12 @@ def _to_datastore(self, entity, data, prefix="", repeated=False):
                 values = [values]
             legacy_values = []
             for value in values:
-                legacy_values.append(
-                    _entity_to_ds_entity(value, set_key=self._keep_keys)
-                )
+                ds_entity = None
+                if value is not None:
+                    ds_entity = _entity_to_ds_entity(
+                        value, set_key=self._keep_keys
+                    )
+                legacy_values.append(ds_entity)
             if not self._repeated:
                 legacy_values = legacy_values[0]
             data[self._name] = legacy_values

packages/google-cloud-ndb/tests/unit/test_model.py

@@ -3756,6 +3756,26 @@ class SubKind(model.Model):
         ds_entity = model._entity_to_ds_entity(entity, set_key=False)
         assert prop._call_from_base_type(ds_entity) == entity
 
+    @staticmethod
+    def test_legacy_optional_local_structured_property(in_context):
+        class SubKind(model.Model):
+            foo = model.Property()
+
+        class ContainerB(model.Model):
+            child_b = model.LocalStructuredProperty(SubKind)
+
+        class ContainerA(model.Model):
+            child_a = model.LocalStructuredProperty(ContainerB)
+
+        with in_context.new(legacy_data=True).use():
+            entity = ContainerA(child_a=ContainerB())
+            data = {"_exclude_from_indexes": []}
+            assert ContainerA.child_a._to_datastore(entity, data) == (
+                "child_a",
+            )
+            assert data.pop("_exclude_from_indexes") == ["child_a"]
+            assert data["child_a"]["child_b"] is None
+
 
 class TestGenericProperty:
     @staticmethod