code: function prepareDocs(coll, docs, options) {
    const forceServerObjectId =
      typeof options.forceServerObjectId === 'boolean'
        ? options.forceServerObjectId
        : coll.s.db.options.forceServerObjectId;
  
    // no need to modify the docs if server sets the ObjectId
    if (forceServerObjectId === true) {
      return docs;
    }
  
    return docs.map(doc => {
      if (forceServerObjectId !== true && doc._id == null) {
        doc._id = coll.s.pkFactory.createPk();
      }
  
      return doc;
    });
  }
docs: modifies documents before being inserted or updated