const Client = require('mongodb');

function CustomPKFactory() {}
CustomPKFactory.prototype = new Object();
CustomPKFactory.count = 0;
CustomPKFactory.createPk = function() {
  return ++CustomPKFactory.count;
};

const host = 'mongodb://localhost:27017';
const options = { pkFactory: CustomPKFactory };

Client.connect(host, options, async (err, client) => {
  const db = client.db('pkFactory');
  const col = db.collection('test_custom_key');
  await col.deleteMany({});

  // ✓ expected _id: 1 | received _id: 1
  await col.insertOne({ a: 1 });

  // ⨉ expected _id: 2 | received _id: ObjectId(…)
  await col.findOneAndUpdate({ a: 2 }, { $set: { b: 2 } }, { upsert: true });

  const bulk = col.initializeUnorderedBulkOp();

  // ⨉ expected _id: 3 | received _id: ObjectId(…)
  bulk.insert({ a: 3 });

  // ⨉ expected _id: 4 | received _id: ObjectId(…)
  bulk
    .find({ a: 4 })
    .upsert()
    .update({ $set: { b: 4 } });

  await bulk.execute();
});

{
    "hosts" : [ 
        "127.0.0.1:3001"
    ],
    "setName" : "pkFactory",
    "setVersion" : 9516979,
    "ismaster" : true,
    "secondary" : false,
    "primary" : "127.0.0.1:27017",
    "me" : "127.0.0.1:27017",
    "electionId" : ObjectId("7fffffff00000000000000a4"),
    "lastWrite" : {
        "opTime" : {
            "ts" : Timestamp(1572097963, 1),
            "t" : NumberLong(164)
        },
        "lastWriteDate" : ISODate("2019-10-26T13:52:43.000Z"),
        "majorityOpTime" : {
            "ts" : Timestamp(1572097963, 1),
            "t" : NumberLong(164)
        },
        "majorityWriteDate" : ISODate("2019-10-26T13:52:43.000Z")
    },
    "maxBsonObjectSize" : 16777216,
    "maxMessageSizeBytes" : 48000000,
    "maxWriteBatchSize" : 100000,
    "localTime" : ISODate("2019-10-26T13:52:44.010Z"),
    "logicalSessionTimeoutMinutes" : 30,
    "minWireVersion" : 0,
    "maxWireVersion" : 7,
    "readOnly" : false,
    "ok" : 1.0,
    "operationTime" : Timestamp(1572097963, 1)
}