Mongoosastic - indexing ObjectId into elastic search

Question

I would like to index my mongoDB objectID when indexing to Elastic Search

Like this :

_id : {type: mongoose.Schema.Types.ObjectId, es_indexed:true},
parentItem : {type: mongoose.Schema.Types.ObjectId, Ref:'SystemItem', es_indexed:true }

This produce undefined when mongoosastic is importing my db into elastic search

There is a way to do this? Or should I go other way?