Auto-generated _id behavior in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the type of the default _id field in MongoDB?

When you insert a document into a MongoDB collection without specifying the _id field, MongoDB automatically generates one. What is the data type of this auto-generated _id?

A An ObjectId, a 12-byte BSON type

B A string representing a UUID

C An integer starting from 1 and incrementing

D A timestamp value only

💡 Hint

Think about the unique identifier MongoDB uses internally for documents.

MongoDB generates an ObjectId by default for the _id field. It is a 12-byte BSON type that includes a timestamp, machine identifier, process id, and a counter to ensure uniqueness.

❓ query_result

intermediate

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

Consider inserting two documents into a MongoDB collection where both documents have the same _id value explicitly set. What will happen?

A Both documents will be inserted successfully with duplicate _id values.

B The second insert will overwrite the first document silently.

C MongoDB will automatically generate a new _id for the second document.

D The second insert will fail with a duplicate key error.

💡 Hint

Remember that _id must be unique in a collection.

The _id field is a unique identifier for documents in a collection. Attempting to insert a document with an _id that already exists causes a duplicate key error.

🧠 Conceptual

advanced

Why does MongoDB use a 12-byte ObjectId for _id by default?

MongoDB's default _id is a 12-byte ObjectId. Which of the following best explains the reason for this design?

A To store a simple incrementing integer for easy sorting

B To embed creation time and ensure uniqueness across machines and processes

C To save space by using a fixed 4-byte integer

D To allow storing large binary data inside the _id field

💡 Hint

Think about what information is useful for a unique identifier in distributed systems.

The 12-byte ObjectId includes a timestamp, machine identifier, process id, and counter. This design ensures uniqueness and encodes creation time for sorting.

📝 Syntax

advanced

Which MongoDB insert command will NOT generate an _id automatically?

Given the following insert commands, which one will NOT cause MongoDB to generate an _id field automatically?

MongoDB

db.collection.insertOne({ name: "Alice", _id: "custom_id_123" })

A db.collection.insertOne({ name: "Alice" })

B db.collection.insertOne({ name: "Alice", _id: ObjectId() })

C db.collection.insertOne({ name: "Alice", _id: "custom_id_123" })

D db.collection.insertOne({ name: "Alice", _id: null })

💡 Hint

When you provide an _id explicitly, MongoDB does not generate one.

If you specify the _id field explicitly in the document, MongoDB uses that value and does not generate a new ObjectId.

🔧 Debug

expert

Why does this MongoDB insert fail with a duplicate key error?

You run the following commands: 1. db.users.insertOne({ name: "Bob" }) 2. db.users.insertOne({ _id: ObjectId("507f1f77bcf86cd799439011"), name: "Carol" }) 3. db.users.insertOne({ _id: ObjectId("507f1f77bcf86cd799439011"), name: "Dave" }) The third insert fails with a duplicate key error. Why?

A Because the _id value in the third insert is the same as the second insert's _id

B Because the first insert generated an _id that conflicts with the third insert

C Because ObjectId values cannot be used as _id fields

D Because the collection does not allow inserting documents with names starting with 'D'

💡 Hint

Check the _id values used in the inserts carefully.

The third insert uses the same explicit _id as the second insert, causing a duplicate key error because _id must be unique.

Practice

1. In MongoDB, what happens if you insert a document without specifying the _id field?

easy

A. MongoDB automatically generates a unique _id for the document.

B. The insert operation fails with an error.

C. The document is inserted with a null _id.

D. MongoDB assigns a sequential integer as the _id.

Solution

1. Step 1: Understand MongoDB's default behavior for _id

   MongoDB requires each document to have a unique _id. If not provided, it creates one automatically.

2. Step 2: Identify the type of auto-generated _id

   The auto-generated _id is an ObjectId, which is unique and generated by MongoDB.

3. Final Answer:

   MongoDB automatically generates a unique _id for the document. -> Option A

4. Quick Check:

   Auto-generated _id = unique ObjectId [OK]

Hint: If no _id, MongoDB creates a unique one automatically [OK]

Common Mistakes:

• Thinking insert fails without _id
• Assuming _id can be null
• Believing _id is a simple number

2. Which of the following is the correct way to insert a document without specifying _id in MongoDB shell?

easy

A. db.collection.insertOne({_id: 1, name: 'Alice'})

B. db.collection.insertOne({_id: null, name: 'Alice'})

C. db.collection.insertOne({name: 'Alice'})

D. db.collection.insertOne()

Solution

1. Step 1: Check syntax for inserting a document without _id

   The correct syntax is to provide the document fields except _id, so MongoDB generates it.

2. Step 2: Evaluate each option

   db.collection.insertOne({name: 'Alice'}) inserts a document with only the name field, letting MongoDB create _id. db.collection.insertOne({_id: null, name: 'Alice'}) sets _id to null which is invalid. db.collection.insertOne({_id: 1, name: 'Alice'}) sets _id manually. db.collection.insertOne() is missing the document argument.

3. Final Answer:

   db.collection.insertOne({name: 'Alice'}) -> Option C

4. Quick Check:

   Insert without _id uses document only [OK]

Hint: Insert document without _id to auto-generate it [OK]

Common Mistakes:

• Passing empty insertOne() without document
• Setting _id to null explicitly
• Confusing manual and automatic _id assignment

3. Consider the following MongoDB shell commands:

db.test.insertOne({name: 'Bob'})
db.test.insertOne({_id: ObjectId('507f1f77bcf86cd799439011'), name: 'Carol'})
db.test.find().count()

What will be the output of the count() command?

medium

A. 2

B. 0

C. 1

D. Error due to duplicate _id

Solution

1. Step 1: Analyze the inserts

   The first insert adds a document without _id, so MongoDB generates one. The second insert adds a document with a specific _id ObjectId.

2. Step 2: Check for duplicates and count documents

   Since the _id in the second insert is unique and different from the first, both inserts succeed. So, the collection has 2 documents.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   Two unique documents inserted = count 2 [OK]

Hint: Unique _id means both inserts succeed [OK]

Common Mistakes:

• Assuming auto-generated _id matches manual one
• Thinking duplicate _id error occurs
• Forgetting count() returns total documents

4. You run this code in MongoDB shell:

db.users.insertOne({_id: 1, name: 'Dave'})
db.users.insertOne({_id: 1, name: 'Eve'})

What will happen and how can you fix it?

medium

A. Second insert overwrites the first document silently.

B. Both inserts succeed; MongoDB allows duplicate _id.

C. First insert fails; _id must be ObjectId.

D. Second insert fails due to duplicate _id; fix by using unique _id values.

Solution

1. Step 1: Understand _id uniqueness constraint

   MongoDB requires _id to be unique in a collection. Duplicate _id values cause insert failure.

2. Step 2: Analyze the inserts

   The first insert with _id: 1 succeeds. The second insert tries the same _id, causing a duplicate key error.

3. Final Answer:

   Second insert fails due to duplicate _id; fix by using unique _id values. -> Option D

4. Quick Check:

   Duplicate _id causes insert failure [OK]

Hint: Each _id must be unique to avoid insert errors [OK]

Common Mistakes:

• Thinking MongoDB allows duplicate _id
• Assuming _id must be ObjectId type
• Believing second insert overwrites first

5. You want to insert multiple documents into a MongoDB collection, but ensure each document has a unique _id without manually specifying it. Which approach correctly achieves this and why?

const docs = [
  {name: 'Anna'},
  {name: 'Ben'},
  {name: 'Cara'}
];
db.collection.insertMany(docs);

hard

A. Insert documents without _id but create a unique index on name.

B. Insert documents as is; MongoDB auto-generates unique _id for each document.

C. Manually assign sequential integers as _id before insert.

D. Add _id: null to each document to let MongoDB generate _id.

Solution

1. Step 1: Understand MongoDB's auto-generation of _id

   When documents lack _id, MongoDB automatically creates a unique ObjectId for each during insert.

2. Step 2: Evaluate each option

   Insert documents as is; MongoDB auto-generates unique _id for each document. correctly relies on MongoDB's default behavior. Add _id: null to each document to let MongoDB generate _id. is invalid because _id: null is not allowed. Manually assign sequential integers as _id before insert. requires manual work and risks duplicates. Insert documents without _id but create a unique index on name. creates a unique index on name, unrelated to _id uniqueness.

3. Final Answer:

   Insert documents as is; MongoDB auto-generates unique _id for each document. -> Option B

4. Quick Check:

   Missing _id means MongoDB creates unique ObjectId [OK]

Hint: Insert without _id to get unique ObjectId automatically [OK]

Common Mistakes:

• Setting _id to null explicitly
• Manually assigning _id unnecessarily
• Confusing unique index on other fields with _id