What do you think of changing `parent_id` to just `_parent`? There's a precedent for the `_parent` field in http://www.elasticsearch.org/guide/reference/api/bulk/. I would have made the `id` field `_id` too, but having a field called `id` is such a common RDBMS idiom that it's worth the exception IMO. (It's also different in that it doesn't pop the field off the doc.)