Fix handling of default values for null nested messages

protarrow/arrow_to_proto.py

@@ -259,7 +259,9 @@ def __iter__(self) -> Iterator[Callable[[pa.Scalar], None]]:
 
     def __call__(self, scalar: pa.Scalar) -> None:
         value = self.converter(scalar) if scalar.is_valid else None
-        if value is not None:
+        # `self.message` can be null for nested messages.
+        # We'd expect the scalar to be either null or the default value in this case
+        if self.message is not None and value is not None:
             if self.nullable:
                 getattr(self.message, self.field_descriptor.name).value = value
             else:
@@ -284,9 +286,10 @@ def __init__(
     def __iter__(self) -> Iterator[Callable[[pa.Scalar], None]]:
         assert self.attribute is None
         for message, size in zip(self.messages, self.sizes):
-            self.attribute = getattr(message, self.field_descriptor.name)
-            for _ in range(size):
-                yield self
+            if message is not None:
+                self.attribute = getattr(message, self.field_descriptor.name)
+                for _ in range(size):
+                    yield self
         self.attribute = None
 
     def __call__(self, scalar: pa.Scalar) -> None:
@@ -312,9 +315,10 @@ def __post_init__(self, key_arrow_type: pa.DataType):
     def __iter__(self) -> Iterator[Callable[[pa.Scalar], Message]]:
         assert self.attribute is None
         for message, offset in zip(self.messages, self.sizes):
-            self.attribute = getattr(message, self.field_descriptor.name)
-            for _ in range(offset):
-                yield self
+            if message is not None:
+                self.attribute = getattr(message, self.field_descriptor.name)
+                for _ in range(offset):
+                    yield self
         self.attribute = None
 
     def __call__(self, scalar: pa.Scalar) -> Message:
@@ -363,9 +367,10 @@ def __post_init__(self, key_arrow_type: pa.DataType, value_arrow_type: pa.DataTy
     def __iter__(self) -> Iterator[Callable[[pa.Scalar, pa.Scalar], Message]]:
         assert self.attribute is None
         for message, size in zip(self.messages, self.sizes):
-            self.attribute = getattr(message, self.field_descriptor.name)
-            for _ in range(size):
-                yield self
+            if message is not None:
+                self.attribute = getattr(message, self.field_descriptor.name)
+                for _ in range(size):
+                    yield self
         self.attribute = None
 
     def __call__(self, key: pa.Scalar, value: pa.Scalar):

tests/test_coverage.py

@@ -312,3 +312,21 @@ def test_missing_enum_from_proto():
     assert protarrow.messages_to_table(
         [message], ExampleMessage, protarrow.ProtarrowConfig(enum_type=pa.binary())
     )["example_enum_value"].to_pylist() == [b"UNKNOWN_EXAMPLE_ENUM"]
+
+
+def test_nested_missing_values():
+    """
+    Check for cases where nested messages are null.
+
+    When a nested message is null, the arrow arrays of their underlying fields
+    """
+    source_messages = [
+        NestedExampleMessage(),
+        NestedExampleMessage(example_message=ExampleMessage(double_value=1.0)),
+    ]
+    table = protarrow.messages_to_table(
+        source_messages,
+        NestedExampleMessage,
+    )
+    messages_back = protarrow.table_to_messages(table, NestedExampleMessage)
+    assert messages_back == source_messages