ARROW-214 discusses adding support for a variable length binary field. It should be more straightforward to implement a fixed-size binary field. For example it should be possible to configure ObjectId as a 12-byte binary:

Just tried to check what I get with an ObjectID extension type if I do field.type.storage_type and I get the pa.binary(12) type as far as I can tell. But if I try to do a schema with {"_id": pa.binary(12)} I get the following error:

Unsupported data type in schema for field "_id" of type "fixed_size_binary[12]"

Is that expected? Should I not be able to specify a schema that casts an ObjectId field directly to its storage type?

Reported here: https://github.com/mongodb-labs/mongo-arrow/issues/236#issuecomment-2378779738