types(model): aligned watch() type for mongodb 4.6.0

[lpizzinidev]

Summary
You can now specify the custom type for the top-level document returned in a change event.
This aligns with the changes introduced in MongoDB 4.6.0.

Example

TestModel.watch<ITest, ChangeStreamInsertDocument<ITest>>([]).on('change', (change) => {
    console.log(change.fullDocument); // TS won't complain
});

Closes #13206

[vkarpov15]

Wouldn't it be better if we added a separate overload for watch() with options: mongodb.ChangeStreamOptions & { fullDocument: 'updateLookup', hydrate?: boolean } that added fullDocument to the result? It looks like this approach relies on the user to specify the correct type

[lpizzinidev]

From my understanding of the changelog, the added feature is that the dev can now specify ChangeType, the type of the change document.
It is not related to the options parameter (I wrote an unclear example above, now edited).

[vkarpov15]

I took a closer look and @lpizzinidev is correct, the expected behavior here is that the user can overwrite the valid change types. fullDocument: 'updateLookup' doesn't always ensure that there will be a fullDocument property, for example deletes. Below is an example script I used to verify:

import mongoose from 'mongoose';

mongoose.set('debug', true);

run().catch(err => console.error(err));

async function run() {
  await mongoose.connect('mongodb://localhost:27017,localhost:27018,localhost:27019/mongoose_test');
  await mongoose.connection.dropDatabase();

  const TestModel = mongoose.model('Test', new mongoose.Schema({ name: String }));
  await TestModel.createCollection();

  await TestModel.
    watch<{name:string}>([], { fullDocument: 'updateLookup' }).
    on('change', change => console.log(change));
  await TestModel.create({ name: 'foo' });
  await TestModel.updateOne({ name: 'foo' }, { name: 'bar' });
  await TestModel.deleteOne({ name: 'bar' });
  console.log('Wrote document');
}