Change stream pipelines for filtering in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Filter change stream to only capture insert operations

Given a MongoDB change stream pipeline, which option correctly filters to capture only insert operations?

MongoDB

const pipeline = [
  { $match: { 'operationType': 'insert' } }
];

const changeStream = collection.watch(pipeline);

A [{ $match: { 'fullDocument': { $exists: false } } }]

B [{ $match: { 'operationType': { $ne: 'insert' } } }]

C [{ $match: { 'operationType': 'insert' } }]

D [{ $match: { 'operationType': 'update' } }]

💡 Hint

Look for the operationType field and match it exactly to 'insert'.

The change stream event document has an 'operationType' field indicating the type of operation. Filtering with $match on 'operationType' equal to 'insert' captures only insert events.

❓ query_result

intermediate

Filter change stream for updates on a specific field

Which pipeline filters a MongoDB change stream to capture only update operations where the field status was changed?

MongoDB

const pipeline = [
  { $match: { 'operationType': 'update', 'updateDescription.updatedFields.status': { $exists: true } } }
];

A [{ $match: { 'operationType': 'update', 'fullDocument.status': { $exists: true } } }]

B [{ $match: { 'operationType': 'update', 'updateDescription.removedFields': 'status' } }]

C [{ $match: { 'operationType': 'insert', 'updateDescription.updatedFields.status': { $exists: true } } }]

D [{ $match: { 'operationType': 'update', 'updateDescription.updatedFields.status': { $exists: true } } }]

💡 Hint

Check the updateDescription.updatedFields object for the changed fields.

Update events include an 'updateDescription' field with 'updatedFields' showing which fields changed. Filtering for 'status' in updatedFields captures updates to that field.

📝 Syntax

advanced

Identify the syntax error in this change stream pipeline

Which option contains a syntax error in the MongoDB change stream pipeline?

MongoDB

const pipeline = [
  { $match: { operationType: 'delete' } },
  { $project: { documentKey: 1, _id: 0 } }
];

A [{ $match: { operationType: 'delete' } }, { $project: { documentKey: 1 _id: 0 } }]

B [{ $match: { 'operationType': 'delete' } }, { $project: { documentKey: 1, _id: 0 } }]

C [{ $match: { operationType: 'delete' } }, { $project: { documentKey: 1, _id: 0 } }]

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💡 Hint

Look carefully at the commas separating fields inside $project.

Option A is missing a comma between 'documentKey: 1' and '_id: 0' inside the $project stage, causing a syntax error.

❓ optimization

advanced

Optimize change stream pipeline to reduce unnecessary data

Which pipeline best optimizes a change stream to only return the _id and changed fields for update operations?

A [{ $match: { operationType: 'update' } }, { $project: { _id: 0 } }]

B [{ $match: { operationType: 'update' } }, { $project: { 'updateDescription.updatedFields': 1, 'documentKey._id': 1, _id: 0 } }]

C [{ $match: { operationType: 'update' } }, { $project: { updateDescription: 1, _id: 0 } }]

D [{ $match: { operationType: 'update' } }, { $project: { fullDocument: 1, _id: 0 } }]

💡 Hint

Focus on projecting only the changed fields and the document ID.

Option B projects only the updated fields and the document's _id, minimizing data sent and improving efficiency.

🧠 Conceptual

expert

Understanding change stream pipeline behavior with multiple $match stages

Consider this change stream pipeline with two $match stages:

[{ $match: { operationType: 'insert' } }, { $match: { 'fullDocument.status': 'active' } }]

What is the effect of using two separate $match stages instead of combining them into one?

A The pipeline filters first for inserts, then filters those inserts for documents where status is 'active', effectively the same as a combined $match.

B The pipeline will fail because multiple $match stages are not allowed in change stream pipelines.

C The pipeline will return all inserts regardless of status because only the first $match is applied.

D The pipeline will only filter by the last $match stage, ignoring the first.

💡 Hint

Think about how MongoDB pipelines process stages sequentially.

MongoDB pipelines process stages in order. Multiple $match stages filter data step-by-step, so two $match stages filter first by operationType, then by status, same as combining conditions in one $match.