Don't Skip Serializing Empty Metadata (#3082)

arrow-schema/Cargo.toml

@@ -45,3 +45,4 @@ default = []
 
 [dev-dependencies]
 serde_json = "1.0"
+postcard = { version = "1.0.2", default-features = false, features = ["alloc"] }

arrow-schema/src/datatype.rs

@@ -415,11 +415,11 @@ mod tests {
         assert_eq!(
             "{\"Struct\":[\
              {\"name\":\"first_name\",\"data_type\":\"Utf8\",\"nullable\":false,\"dict_id\":0,\"dict_is_ordered\":false,\"metadata\":{\"k\":\"v\"}},\
-             {\"name\":\"last_name\",\"data_type\":\"Utf8\",\"nullable\":false,\"dict_id\":0,\"dict_is_ordered\":false},\
+             {\"name\":\"last_name\",\"data_type\":\"Utf8\",\"nullable\":false,\"dict_id\":0,\"dict_is_ordered\":false,\"metadata\":{}},\
              {\"name\":\"address\",\"data_type\":{\"Struct\":\
-             [{\"name\":\"street\",\"data_type\":\"Utf8\",\"nullable\":false,\"dict_id\":0,\"dict_is_ordered\":false},\
-             {\"name\":\"zip\",\"data_type\":\"UInt16\",\"nullable\":false,\"dict_id\":0,\"dict_is_ordered\":false}\
-             ]},\"nullable\":false,\"dict_id\":0,\"dict_is_ordered\":false}]}",
+             [{\"name\":\"street\",\"data_type\":\"Utf8\",\"nullable\":false,\"dict_id\":0,\"dict_is_ordered\":false,\"metadata\":{}},\
+             {\"name\":\"zip\",\"data_type\":\"UInt16\",\"nullable\":false,\"dict_id\":0,\"dict_is_ordered\":false,\"metadata\":{}}\
+             ]},\"nullable\":false,\"dict_id\":0,\"dict_is_ordered\":false,\"metadata\":{}}]}",
             serialized
         );
 

arrow-schema/src/field.rs

@@ -35,10 +35,6 @@ pub struct Field {
     dict_id: i64,
     dict_is_ordered: bool,
     /// A map of key-value pairs containing additional custom meta data.
-    #[cfg_attr(
-        feature = "serde",
-        serde(skip_serializing_if = "BTreeMap::is_empty", default)
-    )]
     metadata: BTreeMap<String, String>,
 }
 
@@ -603,4 +599,37 @@ mod test {
         assert!(!field1.contains(&field2));
         assert!(!field2.contains(&field1));
     }
+
+    #[cfg(feature = "serde")]
+    fn assert_binary_serde_round_trip(field: Field) {
+        let serialized = postcard::to_allocvec(&field).unwrap();
+        let deserialized: Field = postcard::from_bytes(&serialized).unwrap();
+        assert_eq!(field, deserialized)
+    }
+
+    #[cfg(feature = "serde")]
+    #[test]
+    fn test_field_without_metadata_serde() {
+        let field = Field::new("name", DataType::Boolean, true);
+        assert_binary_serde_round_trip(field)
+    }
+
+    #[cfg(feature = "serde")]
+    #[test]
+    fn test_field_with_empty_metadata_serde() {
+        let field =
+            Field::new("name", DataType::Boolean, false).with_metadata(BTreeMap::new());
+
+        assert_binary_serde_round_trip(field)
+    }
+
+    #[cfg(feature = "serde")]
+    #[test]
+    fn test_field_with_nonempty_metadata_serde() {
+        let mut metadata = BTreeMap::new();
+        metadata.insert("hi".to_owned(), "".to_owned());
+        let field = Field::new("name", DataType::Boolean, false).with_metadata(metadata);
+
+        assert_binary_serde_round_trip(field)
+    }
 }

arrow-schema/src/schema.rs

@@ -34,10 +34,6 @@ pub type SchemaRef = std::sync::Arc<Schema>;
 pub struct Schema {
     pub fields: Vec<Field>,
     /// A map of key-value pairs containing additional meta data.
-    #[cfg_attr(
-        feature = "serde",
-        serde(skip_serializing_if = "HashMap::is_empty", default)
-    )]
     pub metadata: HashMap<String, String>,
 }