C100DEV Practice Test 6 — 10 Questions

Question 1

When designing a schema for a blog application, you need to store comments for posts. The comments grow indefinitely. Which approach is most effective for long-term scalability?

Accepted Answer

Use the Hybrid pattern for embedded and referenced comments.. The Hybrid pattern is best here. You embed the most recent comments (e.g., the last 10) directly within the post document to allow for fast initial page loads. Simultaneously, you store older comments in a separate collection with a reference to the post ID. This provides a balance between high-speed access for recent content and the ability to scale indefinitely without ever hitting the BSON size limit.

Answer

Embed all comments in the post document.

Answer

Reference all comments in a separate collection.

Answer

Use the Attribute pattern for every comment.

Question 2

A developer wants to remove all documents from the 'logs' collection where the 'level' field equals 'debug'. The collection has an index on level. Which operation should be used, and what does it return?

Accepted Answer

deleteMany({ level: 'debug' }) returns a result whose deletedCount is the number of documents removed.. deleteMany with a filter removes all matching documents and reports how many were deleted via deletedCount. deleteOne only removes the first match, drop destroys the whole collection, and remove is deprecated. When a selective deletion is needed and the filter is indexed, deleteMany preserves unrelated documents and existing indexes.

Answer

drop() removes the collection and its index, which is the recommended way to clear matching documents.

Answer

deleteOne({ level: 'debug' }) returns the number of documents removed.

Answer

remove({ level: 'debug' }) is the modern method and returns a cursor of deleted documents.

Question 3

Refer to the exhibit. Why would an operation attempting to insert the document shown in the exhibit fail?

Accepted Answer

The field '$created' starts with a reserved character.. The document contains a field named '$created'. In MongoDB, field names starting with a '$' symbol are reserved for internal query operators. When the database engine parses a document, it interprets fields beginning with '$' as instructions rather than data fields. This naming conflict is a common trap for developers migrating legacy data or building generic metadata structures without validating field name constraints.

Answer

The '_id' field is invalid.

Answer

The field 'metadata' is a reserved keyword.

Answer

The value 'null' is not allowed for the 'last_login' field.

Question 4

You are designing an e-commerce catalog where products can have a dynamic array of tags. You need to query for documents where the 'tags' array contains both 'electronics' and 'sale' simultaneously, regardless of their order or other elements. Which query operator should you use?

Accepted Answer

{ tags: { $all: ['electronics', 'sale'] } }. The $all operator selects the documents where the value of a field is an array that contains all the specified elements. This is essential when searching arrays for multiple matching criteria without enforcing strict positional indexing or exact array equality.

Answer

{ tags: { $in: ['electronics', 'sale'] } }

Answer

{ tags: ['$electronics', '$sale'] }

Answer

{ tags: { $elemMatch: { $eq: 'electronics', $eq: 'sale' } } }

Question 5

Which schema pattern is demonstrated in the exhibit provided?

Accepted Answer

The Embedding Pattern.. The exhibit shows an array of sub-documents nested directly within a parent document, which is the definition of the embedding pattern. This pattern is used to keep related data together in one physical storage unit. It is highly efficient for read-heavy applications where the parent user record and their order history are almost always required at the same time by the application layer.

Answer

The Linking Pattern.

Answer

The Polymorphic Pattern.

Answer

The Computed Pattern.

Question 6

When using the MongoDB C#/.NET driver, which object is essential for managing the scope and lifecycle of a multi-document transaction?

Accepted Answer

IClientSessionHandle. In the .NET driver, the 'IClientSessionHandle' object is used to represent a session that controls the lifecycle of a transaction. By using the 'StartTransaction' method on this session object, developers can encapsulate multiple operations within a single, atomic unit of work. This session handle is passed to every database operation within the transaction, ensuring that all actions are associated with the correct transactional scope and are properly committed or aborted.

Answer

IMongoDatabase

Answer

MongoClient

Answer

IMongoCollection

Question 7

A MongoDB application stores inventory documents in the 'inventory' collection with fields 'sku', 'on_hand', and 'reserved'. During a flash sale, many concurrent sessions must decrement 'on_hand' by 1 only when on_hand is greater than 0, and each successful decrement must return the resulting document to the client. Which approach guarantees this behavior?

Accepted Answer

db.inventory.findOneAndUpdate({ sku: 'A100', on_hand: { $gt: 0 } }, { $inc: { on_hand: -1 } }, { returnDocument: 'after' }). findOneAndUpdate performs the conditional match, the $inc decrement, and the return of the document in one atomic operation. The on_hand: { $gt: 0 } filter guards against negative inventory under concurrency, and returnDocument: 'after' supplies the resulting state. Separating read and write, or using updateOne or bulkWrite, either introduces races or fails to return the document.

Answer

db.inventory.bulkWrite([{ updateOne: { filter: { sku: 'A100', on_hand: { $gt: 0 } }, update: { $inc: { on_hand: -1 } } } }])

Answer

db.inventory.find({ sku: 'A100', on_hand: { $gt: 0 } }) followed by db.inventory.updateOne({ sku: 'A100' }, { $inc: { on_hand: -1 } })

Answer

db.inventory.updateOne({ sku: 'A100', on_hand: { $gt: 0 } }, { $inc: { on_hand: -1 } }, { returnDocument: 'after' })

Question 8

A developer needs to perform a point-in-time backup of a MongoDB replica set while the database remains online and serving reads and writes. Which MongoDB Database Tool is designed for this task?

Accepted Answer

mongodump with --oplog. mongodump with --oplog is the MongoDB Database Tool that captures both data and oplog entries during a live dump, enabling point-in-time restore. It runs against a running replica set without downtime. mongoexport lacks oplog capture, mongorestore applies dumps rather than creating them, and mongostat is a monitoring tool, so none of those meet the backup requirement.

Answer

mongostat with --discover

Answer

mongorestore with --archive

Answer

mongoexport with --query

Question 9

A developer wants to query MongoDB Atlas Search for documents where the 'description' field contains the phrase 'wireless headphones'. Which Atlas Search operator should be used to achieve this?

Accepted Answer

The phrase operator.. To search for an exact phrase in Atlas Search, the phrase operator is the correct choice. It ensures that the terms appear in the specified order and adjacent to each other. Other operators like text, wildcard, or compound serve different purposes and would not guarantee phrase matching.

Answer

The compound operator.

Answer

The wildcard operator.

Answer

The text operator.

Question 10

A social media platform stores posts in a collection named posts. Each post document contains an array field `comments`, where each comment is a subdocument with fields `user` and `text`. The development team needs to generate a report that lists each comment as a separate document, including the post's `_id` and the comment's `user` and `text`. Which aggregation stage should be used to achieve this?

Accepted Answer

$unwind. The $unwind stage is specifically designed to deconstruct an array field, outputting one document per array element. In this case, unwinding the `comments` array will produce a document for each comment, retaining the parent post's `_id`. This is the correct stage to flatten the comments array and meet the reporting requirement.

Answer

$project

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$replaceRoot

Answer

$group