Auto-generated _id behavior in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to insert a document and let MongoDB auto-generate the _id.

MongoDB

db.collection.insertOne({ name: "Alice", [1] })

Drag options to blanks, or click blank then click option'

A_id: 12345

Bname: "Bob"

Cage: 25

D_id: ObjectId()

Attempts:

3 left

💡 Hint

Common Mistakes

Including _id manually when you want it auto-generated.

Using a fixed _id value which may cause duplicate key errors.

When inserting a document without specifying _id, MongoDB auto-generates it. So you should not include _id in the document to let it be auto-generated.

2fill in blank

medium

Complete the code to find a document by its auto-generated _id.

MongoDB

db.collection.findOne({ _id: [1] })

Drag options to blanks, or click blank then click option'

A"507f1f77bcf86cd799439011"

BObjectId("507f1f77bcf86cd799439011")

C12345

D"Alice"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the _id string without ObjectId() wrapper.

Using a number or other type instead of ObjectId.

MongoDB stores _id as an ObjectId type, so you must use ObjectId("...") to query by it.

3fill in blank

hard

Fix the error in the code to insert a document with a custom _id.

MongoDB

db.collection.insertOne({ [1]: 1001, name: "Bob" })

Drag options to blanks, or click blank then click option'

A_id

Bid

CID

DobjectId

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'id' or 'ID' instead of '_id'.

Misspelling the field name.

The field for the unique identifier in MongoDB documents is _id (with an underscore). Using any other name will not set the document's _id.

4fill in blank

hard

Fill both blanks to create a document with a custom _id and insert it.

MongoDB

db.collection.insertOne({ [1]: [2], name: "Carol" })

Drag options to blanks, or click blank then click option'

A_id

B1002

Cid

D"1002"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'id' instead of '_id'.

Putting quotes around the number when a number is intended.

To set a custom _id, use the field name '_id' and assign it a value like 1002 (number or string). Here, 1002 as a number is used.

5fill in blank

hard

Fill all three blanks to query documents with auto-generated _id and project only the name field.

MongoDB

db.collection.find({ [1]: { $exists: true } }, { [2]: 1, [3]: 0 })

Drag options to blanks, or click blank then click option'

A_id

Bname

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong field names in query or projection.

Including and excluding the same field in projection incorrectly.

This query finds documents where _id exists (which is always true), and projects only the name field by including it (1) and excluding _id (0) from the output.

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