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.