Creating own "_id" field in MongoDB collection

Descripition

How can I create a valid "_id" field in a struct that MongoDB accepts for its default primary key field? I'm having this trouble when trying to insert a structure into a MongoDB database as a collection using the Database Toolbox Interface for MongoDB. Since MongoDB automatically creates a hexadecimal number for "_id" field for each document even if I give it an "ID" field and I don't want the "_id" to be a hexadecimal (I'd like to provide certain numbers from MATLAB) or having both "_id" and "ID" fields, I should have an "_id" field with the desired value in the structure I'm passing on to the interface.

Errors I got

According to the MATLAB documentation, a valid field name should start with a letter, so when running

cellProps = struct('_id', []);

I get the following error:

Error using struct
Invalid field name "_id".

Steps that did not solve my problem

Checking how MATLAB displays the "_id" field in the Command Window querying a MongoDB database (importing records). It resulted "x_id", and when giving this name to the exported structure's field, MongoDB still creates both "_id" (hexadecimal) and my own "x_id".

[Jeff]

I think that's a proprietary ID used only by MongoDb. I'm trying to figure out how to read it, because matlab won't let me address it as "thing._id" ;)

[Jason Steiner]

Reading it requires a slight workaround, but it is pretty painless. You need to reference that field in paranthesis mode.

id = thing.('_id');

I'm still fighting the original issue from this post; I can't set the _id term in the write, so I can't update a collection that has multiple documents that match the queries needed.