How can we modify documents?

Update operations modify the fields and values of a document while keeping other fields and values unchanged. Replace operations substitute all fields and values in an existing document with specified fields and values while keeping the _id field value unchanged.

The Node.js driver provides the following methods to change documents:

1
updateOne()
2
updateMany()
3
replaceOne()

The top level of an update document contains one or more of the following update operators:

1
$set: replaces the value of a field with a specified one
2
$inc: increments or decrements field values
3
$rename: renames fields
4
$unset: removes fields
5
$mul: multiplies a field value by a specified number

javascript

Difficulty: 5/10

Topics: update operators, upserts & atomicity, schema migration

Scenario Questions

0-2 years experience

1
We have a collection of user profiles, each document has a 'loginCount' field. How would you increment that field each time the user logs in?
2
A bug requires you to rename the field 'addr' to 'address' in all existing documents. Which MongoDB update operator would you use and how?
3
If you need to add a new field 'isActive' with default true to a single document identified by its _id, how would you write that update?

2-5 years experience

1
Our service needs to add a new tag to an array field 'tags' for all products that match a certain category, but we must avoid duplicate tags. How would you construct the update?
2
During a rollout we discovered that some orders are missing a 'status' field. We want to set it to 'pending' for those documents, but only if the field doesn't exist. How would you implement this in MongoDB, and what considerations are there for concurrency?
3
We attempted an update with upsert:true but observed that duplicate documents were created under high load. Walk me through why that might happen and how you'd fix it.

5-8 years experience

1
Our analytics pipeline needs to migrate a large collection from storing timestamps as strings to ISODate objects. Describe how you would perform this migration with minimal impact on read/write latency.
2
We have a sharded cluster and need to run a bulk update that modifies a field used as a shard key. What are the risks and how would you approach this operation?
3
Explain how you would use MongoDB's change streams to audit document modifications in a high‑throughput microservice, and what performance trade‑offs you would consider.

8+ years experience

1
Our organization is moving from a monolithic MongoDB deployment to a multi‑region, polyglot persistence architecture. How would you design a strategy for evolving document schemas across services while ensuring backward compatibility?
2
We need to deprecate a field used by several legacy services without causing downtime. Outline a phased migration plan, including how you’d handle versioning and data validation.
3
Discuss the pros and cons of using server‑side scripts (e.g., $where) versus application‑level logic for complex document modifications at scale.

Follow-up Questions

❓What could go wrong if you forget to include the upsert option in that scenario?
❓How would you verify that your bulk migration didn't introduce data inconsistencies?
❓Can you describe a way to make the update safe under high concurrent writes?