Using Primary Keys with React Native

I am using react navigation and want to update an entry in a subscreen. The problem is that I can’t pass the actual “project” object because the key is BSON and can’t be serialized. So I thought I would search by primarykey but not sure what to search for. The boilerplate code defines primarykey in schema and apparently sets it to the _id. The question is what is the primaryKey really set to? I would like to be able to have it be some value that can be serialized or even better the HexString of the _id so I can determine the primaryKey from the _id. Thoughts?

class Project extends Realm.Object { 
    static generate(title, description, address, lat, lon) {  
      return {
        _id: new Realm.BSON.ObjectId(),
        title , 
        createdAt: new Date(),

    static schema = {
      name: 'Project',
      primaryKey: '_id',
      properties: {
        _id: 'objectId',
        title: 'string',
        description: 'string', 
        lat: 'float',
        lon: 'float',
        createdAt: 'date'