How to access _id from document in rust

I am currently building a rust app which relyes on mongodb. I am struggeling to create the struct for documents without manuanlly setting them. I need to be able to access the _id. I am currently using the bson::default() however, I am conserned that the Id may be already used within the database. Is this a correct approch or is there a diffrent way to access the _id while letting the mongo driver assign an id.
Thank you for reading.