How to watch for changes to specific fields in MongoDB change stream

Question

I am using the node driver for mongodb to initiate a change stream on a document that has lots of fields that update continuously (via some logic on the insert/update end that c

Answer 1

I'm looking for something similar, but from the blog post at https://www.mongodb.com/blog/post/an-introduction-to-change-streams, it looks like you might need to change your filter to:

var filter = {
    $match: {
        $and: [
            { "updateDescription.updatedFields.SomeFieldA": { $exists: true } },
            { operationType: 'update'}
        ]
    }
};

Answer 2

So i figured this out...

For anyone else interested: My "pipeline" (filter, in my example) needs to be an array

this works...

const MongoClient = require('mongodb').MongoClient;

const uri = 'mongodb://localhost:27017/?replicaSet=rs0';
MongoClient.connect(uri, function(err, client) {

    const db = client.db('mydb');
    // Connect using MongoClient
    var filter = [{
        $match: {
            $and: [
                { "updateDescription.updatedFields.SomeFieldA": { $exists: true } },
                { operationType: "update" }]
        }
    }];

    var options = { fullDocument: 'updateLookup' };
    db.collection('somecollection').watch(filter, options).on('change', data => 
    {
        console.log(new Date(), data);
    });
});