viirya commented on code in PR #7865:
URL: https://github.com/apache/arrow-rs/pull/7865#discussion_r2187517979


##########
parquet-variant/src/builder.rs:
##########
@@ -668,43 +739,41 @@ impl<'a, 'b> ObjectBuilder<'a, 'b> {
         self
     }
 
-    /// Return a new [`ObjectBuilder`] to add a nested object with the 
specified
-    /// key to the object.
-    pub fn new_object(&mut self, key: &'b str) -> ObjectBuilder {
-        self.check_pending_field();
-
-        let field_start = self.buffer.offset();
-        let obj_builder = ObjectBuilder::new(&mut self.buffer, 
self.metadata_builder)
-            .with_validate_unique_fields(self.validate_unique_fields);
-        self.pending = Some((key, field_start));
-
-        obj_builder
+    // Returns validate_unique_fields because we can no longer reference self 
once this method returns.
+    fn parent_state<'b>(&'b mut self, key: &'b str) -> (ParentState<'b>, bool) 
{
+        let state = ParentState::Object {
+            buffer: &mut self.buffer,
+            metadata_builder: self.parent_state.metadata_builder(),
+            fields: &mut self.fields,
+            field_name: key,
+        };
+        (state, self.validate_unique_fields)
     }
 
-    /// Return a new [`ListBuilder`] to add a list with the specified key to 
the
-    /// object.
-    pub fn new_list(&mut self, key: &'b str) -> ListBuilder {
-        self.check_pending_field();
-
-        let field_start = self.buffer.offset();
-        let list_builder = ListBuilder::new(&mut self.buffer, 
self.metadata_builder)
-            .with_validate_unique_fields(self.validate_unique_fields);
-        self.pending = Some((key, field_start));
-
-        list_builder
+    /// Returns an object builder that can be used to append a new (nested) 
object to this list.
+    ///
+    /// WARNING: The builder will have no effect unless/until 
[`ObjectBuilder::finish`] is called.
+    pub fn new_object<'b>(&'b mut self, key: &'b str) -> ObjectBuilder<'b> {
+        let (parent_state, validate_unique_fields) = self.parent_state(key);
+        ObjectBuilder::new(parent_state, validate_unique_fields)
     }
 
-    /// Finalize object
+    /// Returns a list builder that can be used to append a new (nested) list 
to this list.
     ///
-    /// This consumes self and writes the object to the parent buffer.
-    pub fn finish(mut self) -> Result<(), ArrowError> {
-        self.check_pending_field();
+    /// WARNING: The builder will have no effect unless/until 
[`ListBuilder::finish`] is called.
+    pub fn new_list<'b>(&'b mut self, key: &'b str) -> ListBuilder<'b> {
+        let (parent_state, validate_unique_fields) = self.parent_state(key);
+        ListBuilder::new(parent_state, validate_unique_fields)
+    }
 
+    /// Finalizes this object and appends it to its parent, which otherwise 
remains unmodified.
+    pub fn finish(mut self) -> Result<(), ArrowError> {
+        let metadata_builder = self.parent_state.metadata_builder();

Review Comment:
   > The new code doesn't insert a new object field's name to the metadata 
dictionary until the builder's finishmethod is called.
   
   Doesn't it? I saw that `insert` already modify the metadata builder. It is 
happened before `finish` is called, right?
   
   
   ```rust
   pub fn insert<'m, 'd, T: Into<Variant<'m, 'd>>>(&mut self, key: &str, value: 
T) {
     let metadata_builder = self.parent_state.metadata_builder();
     let field_id = metadata_builder.upsert_field_name(key); // <- it is 
inserted into the metadata builder
   }
   ```
   
   The only difference to previous code, is `check_pending_field` is removed 
now so if it is not finished, the field won't be updated to `fields` and no 
effect to `finish`.
   



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