Custom _id values in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What will be the _id of the inserted document?

Consider the following MongoDB insert operation where a custom _id is provided:

db.users.insertOne({ _id: "user123", name: "Alice" })

What will be the value of the _id field in the inserted document?

MongoDB

db.users.insertOne({ _id: "user123", name: "Alice" })

A An ObjectId generated automatically

B null

C "user123"

D A numeric value starting from 1

💡 Hint

If you provide _id explicitly, MongoDB uses that value instead of generating one.

When you insert a document with a custom _id, MongoDB uses that exact value as the document's _id instead of creating a new ObjectId.

❓ query_result

intermediate

What happens if you insert two documents with the same custom _id?

Given the following two insert operations:

db.products.insertOne({ _id: 101, name: "Pen" })
db.products.insertOne({ _id: 101, name: "Pencil" })

What will happen when the second insert runs?

MongoDB

db.products.insertOne({ _id: 101, name: "Pen" })
db.products.insertOne({ _id: 101, name: "Pencil" })

A The second insert will fail with a duplicate key error

B The second document will overwrite the first one

C Both documents will be inserted successfully

D MongoDB will automatically change the _id of the second document

💡 Hint

The _id field must be unique in a collection.

MongoDB enforces uniqueness on the _id field. Inserting a document with an existing _id causes a duplicate key error.

📝 Syntax

advanced

Which insert command correctly sets a custom _id as an ObjectId?

You want to insert a document with a custom _id that is an ObjectId value. Which of the following commands is correct?

A db.orders.insertOne({ _id: ObjectId, item: "Book" })

B db.orders.insertOne({ _id: ObjectId("507f1f77bcf86cd799439011"), item: "Book" })

C db.orders.insertOne({ _id: new ObjectId(), item: "Book" })

D db.orders.insertOne({ _id: "ObjectId(\"507f1f77bcf86cd799439011\")", item: "Book" })

💡 Hint

ObjectId needs to be called as a function with a string argument.

Option B correctly calls ObjectId with a string to create an ObjectId value. Option B uses a string literal, not an ObjectId. Option B misses parentheses. Option B uses the ObjectId function itself, not a value.

❓ optimization

advanced

What is a benefit of using custom _id values instead of default ObjectIds?

Which of the following is a valid advantage of using custom _id values in MongoDB documents?

A You can use meaningful keys that improve query readability and indexing

B MongoDB will automatically optimize queries better with custom _id

C Custom _id values reduce storage size compared to ObjectIds

D Custom _id values allow multiple documents with the same _id

💡 Hint

Think about how meaningful keys can help when searching or indexing.

Using custom _id values lets you assign meaningful identifiers, which can make queries easier to understand and sometimes improve indexing strategies. MongoDB does not automatically optimize queries better just because _id is custom. Custom _id values do not reduce storage size necessarily, and _id must always be unique.

🧠 Conceptual

expert

Why might using a sequential integer as a custom _id cause performance issues?

Suppose you use sequential integers (1, 2, 3, ...) as custom _id values in a large MongoDB collection. What is a likely downside of this approach?

A It will cause duplicate key errors automatically

B MongoDB does not support integer _id values

C Sequential integers cause documents to be stored out of order

D It can cause index hotspots leading to slower write performance

💡 Hint

Think about how indexes handle inserts with increasing keys.

Using sequential _id values causes all inserts to target the same end of the index, creating hotspots that can slow down writes. MongoDB supports integer _id values. Documents are stored in insertion order, and duplicates only occur if you reuse _id values.