fix: parallel arrow with differing schema

bindings/python/pymongoarrow/api.py

@@ -169,12 +169,12 @@ def args_iterable():
     if parallelism == "threads":
         with ThreadPoolExecutor(max_workers=4) as executor:
             results = list(executor.map(lambda args: _process_batch(*args), args_iterable()))
-        return pa.concat_tables(results, promote_options="default")
+        return pa.concat_tables(results, promote_options="permissive")
 
     if parallelism == "processes":
         with multiprocessing.Pool(processes=4) as pool:
             results = pool.starmap(_process_batch, args_iterable())
-        return pa.concat_tables(results, promote_options="default")
+        return pa.concat_tables(results, promote_options="permissive")
 
     context = PyMongoArrowContext(
         schema, codec_options=collection.codec_options, allow_invalid=allow_invalid

bindings/python/pymongoarrow/lib.pyx

@@ -244,7 +244,16 @@ cdef class BuilderManager:
             builder = <_ArrayBuilderBase>self.builder_map.get(full_key, None)
             # If the inferred type was int32 but the same field has an int64 value,
             # re-infer the field's type since int32 is a strict subset of int64.
-            if not self.has_schema and (builder is None or builder.type_marker == BSON_TYPE_INT32 and value_t == BSON_TYPE_INT64):
+            # If builder already existed, avoid ditching previously appended values.
+            if not self.has_schema and builder is not None and builder.type_marker == BSON_TYPE_INT32 and value_t == BSON_TYPE_INT64:
+                old_array = builder.finish().cast('int64')
+                builder = self.get_builder(full_key, value_t, doc_iter, True)
+                for val in old_array:
+                    if val.is_valid:
+                        (<Int64Builder>builder).builder.get().Append(val)
+                    else:
+                        (<Int64Builder>builder).builder.get().AppendNull()
+            elif not self.has_schema and builder is None:
                 builder = self.get_builder(full_key, value_t, doc_iter, True)
             if builder is None:
                 continue

bindings/python/test/test_arrow.py

@@ -1401,3 +1401,61 @@ def test_find_arrow_all_parallelism_options(self):
             table_off.equals(table_thread),
             msg=f"tables differ:\n{table_off}\n\n{table_thread}",
         )
+
+    def test_find_multiple_batches_of_different_schema(self):
+        docs = [{"_id": i, "value": i} for i in range(50)] + [
+            {
+                "_id": 50,
+                "value": 2**40,  # Value much larger than Int32 max
+            }
+        ]
+        self.coll.insert_many(docs)
+
+        orig_method = self.coll.find_raw_batches
+
+        def mock_find_raw_batches(*args, **kwargs):
+            kwargs["batch_size"] = 10
+            return orig_method(*args, **kwargs)
+
+        with mock.patch.object(
+            pymongo.collection.Collection,
+            "find_raw_batches",
+            wraps=mock_find_raw_batches,
+        ):
+            table_off = find_arrow_all(
+                self.coll,
+                {},
+                parallelism="off",
+            )
+            table_proc = find_arrow_all(
+                self.coll,
+                {},
+                parallelism="processes",
+            )
+            table_thread = find_arrow_all(
+                self.coll,
+                {},
+                parallelism="threads",
+            )
+
+        self.assertEqual(table_off.num_rows, len(docs))
+        self.assertEqual(table_proc.num_rows, len(docs))
+        self.assertEqual(table_thread.num_rows, len(docs))
+
+        self.assertTrue(
+            table_off.schema.equals(table_proc.schema),
+            msg=f"{table_off.schema} != {table_proc.schema}",
+        )
+        self.assertTrue(
+            table_off.schema.equals(table_thread.schema),
+            msg=f"{table_off.schema} != {table_thread.schema}",
+        )
+
+        self.assertTrue(
+            table_off.equals(table_proc),
+            msg=f"tables differ:\n{table_off}\n\n{table_proc}",
+        )
+        self.assertTrue(
+            table_off.equals(table_thread),
+            msg=f"tables differ:\n{table_off}\n\n{table_thread}",
+        )