fix: allow for legacy repeated structured properties with empty values

google/cloud/ndb/model.py

@@ -4302,23 +4302,24 @@ def _to_datastore(self, entity, data, prefix="", repeated=False):
         if not self._repeated:
             values = (values,)
 
-        props = tuple(_properties_of(*values))
+        if values:
+            props = tuple(_properties_of(*values))
 
-        for value in values:
-            if value is None:
-                keys.extend(
-                    super(StructuredProperty, self)._to_datastore(
-                        entity, data, prefix=prefix, repeated=repeated
+            for value in values:
+                if value is None:
+                    keys.extend(
+                        super(StructuredProperty, self)._to_datastore(
+                            entity, data, prefix=prefix, repeated=repeated
+                        )
                     )
-                )
-                continue
+                    continue
 
-            for prop in props:
-                keys.extend(
-                    prop._to_datastore(
-                        value, data, prefix=next_prefix, repeated=next_repeated
+                for prop in props:
+                    keys.extend(
+                        prop._to_datastore(
+                            value, data, prefix=next_prefix, repeated=next_repeated
+                        )
                     )
-                )
 
         return set(keys)
 

tests/system/test_crud.py

@@ -1164,6 +1164,33 @@ class SomeKind(ndb.Model):
         assert isinstance(retrieved.bar[2], OtherKind)
 
 
+@pytest.mark.usefixtures("client_context")
+def test_legacy_repeated_structured_property_w_expando_empty(
+    ds_client, dispose_of, client_context
+):
+    """Regression test for #669
+
+    https://github.com/googleapis/python-ndb/issues/669
+    """
+
+    class OtherKind(ndb.Expando):
+        one = ndb.StringProperty()
+
+    class SomeKind(ndb.Model):
+        foo = ndb.IntegerProperty()
+        bar = ndb.StructuredProperty(OtherKind, repeated=True)
+
+    entity = SomeKind(foo=42, bar=[])
+
+    with client_context.new(legacy_data=True).use():
+        key = entity.put()
+        dispose_of(key._key)
+
+        retrieved = key.get()
+        assert retrieved.foo == 42
+        assert retrieved.bar == []
+
+
 @pytest.mark.usefixtures("client_context")
 def test_insert_expando(dispose_of):
     class SomeKind(ndb.Expando):

tests/unit/test_model.py

@@ -3524,6 +3524,20 @@ class SomeKind(model.Model):
             assert SomeKind.foo._to_datastore(entity, data) == {"foo.bar"}
             assert data == {"foo.bar": ["baz", "boz"]}
 
+    @staticmethod
+    def test__to_datastore_legacy_repeated_empty_value(in_context):
+        class SubKind(model.Model):
+            bar = model.Property()
+
+        class SomeKind(model.Model):
+            foo = model.StructuredProperty(SubKind, repeated=True)
+
+        with in_context.new(legacy_data=True).use():
+            entity = SomeKind(foo=[])
+            data = {}
+            assert SomeKind.foo._to_datastore(entity, data) == set()
+            assert data == {}
+
     @staticmethod
     def test__prepare_for_put():
         class SubKind(model.Model):