createIndex method in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an ascending index on the "name" field.

MongoDB

db.collection.createIndex({ name: [1] })

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

A1

B-1

C0

D"asc"

Attempts:

3 left

💡 Hint

Common Mistakes

Using -1 instead of 1 for ascending index.

Using 0 which is invalid for index direction.

Using string "asc" which is not accepted.

The value 1 specifies an ascending index in MongoDB.

2fill in blank

medium

Complete the code to create a unique index on the "email" field.

MongoDB

db.users.createIndex({ email: 1 }, { [1]: true })

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

AexpireAfterSeconds

Bsparse

Cunique

Dbackground

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'sparse' which only indexes documents with the field.

Using 'background' which controls index build behavior.

Using 'expireAfterSeconds' which is for TTL indexes.

The unique option ensures no duplicate values in the indexed field.

3fill in blank

hard

Fix the error in the code to create a descending index on the "date" field.

MongoDB

db.events.createIndex({ date: [1] })

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

Aasc

B0

Cdesc

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using strings like 'asc' or 'desc' which are invalid.

Using 0 which does not specify direction.

Use -1 to specify a descending index in MongoDB.

4fill in blank

hard

Fill both blanks to create a TTL index on the "createdAt" field that expires documents after 3600 seconds.

MongoDB

db.logs.createIndex({ createdAt: [1] }, { [2]: 3600 })

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

A1

BexpireAfterSeconds

Cunique

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using descending index (-1) for TTL which is allowed but less common.

Using 'unique' option which is unrelated to TTL.

The index direction is 1 (ascending), and expireAfterSeconds sets the TTL duration.

5fill in blank

hard

Fill all three blanks to create a compound index on "lastName" (ascending) and "age" (descending) with background building enabled.

MongoDB

db.people.createIndex({ lastName: [1], age: [2] }, { [3]: true })

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

A1

B-1

Cbackground

Dunique

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'unique' instead of 'background' for the option.

Mixing up ascending and descending values.

The compound index uses ascending (1) for lastName, descending (-1) for age, and builds in the background with background: true.

Practice

1. What is the main purpose of the createIndex method in MongoDB?

easy

A. To speed up queries by creating an index on a field

B. To delete documents from a collection

C. To update multiple documents at once

D. To create a new collection

Solution

1. Step 1: Understand what createIndex does

   The createIndex method organizes data to make searches faster.

2. Step 2: Identify the correct purpose

   It does not delete or update documents, nor create collections, only speeds up queries.

3. Final Answer:

   To speed up queries by creating an index on a field -> Option A

4. Quick Check:

   createIndex speeds up queries [OK]

Hint: Remember: createIndex speeds up searches, not data changes [OK]

Common Mistakes:

• Confusing createIndex with delete or update operations
• Thinking createIndex creates collections
• Assuming createIndex modifies documents

2. Which of the following is the correct syntax to create an ascending index on the field age in MongoDB?

easy

A. db.collection.createIndex({ age: 1 })

B. db.collection.createIndex(age: 1)

C. db.collection.createIndex({ age: -1 })

D. db.collection.createIndex({ age })

Solution

1. Step 1: Recall syntax for createIndex

   The method requires an object with field names as keys and 1 or -1 as values for ascending or descending.

2. Step 2: Identify correct ascending index syntax

   { age: 1 } means ascending index on age. Other options are either descending (-1) or invalid syntax.

3. Final Answer:

   db.collection.createIndex({ age: 1 }) -> Option A

4. Quick Check:

   Ascending index uses 1 [OK]

Hint: Use 1 for ascending, -1 for descending in createIndex [OK]

Common Mistakes:

• Using -1 for ascending index
• Missing curly braces around field and order
• Passing field without object braces

3. Given the command db.users.createIndex({ email: 1 }, { unique: true }), what will happen if you try to insert two documents with the same email value?

medium

A. The index will be ignored and duplicates allowed

B. Both documents will be inserted without error

C. The second insert will fail due to duplicate key error

D. MongoDB will automatically update the first document

Solution

1. Step 1: Understand unique index effect

   Unique index prevents duplicate values in the indexed field.

2. Step 2: Predict behavior on duplicate insert

   Inserting a duplicate email will cause an error and reject the second insert.

3. Final Answer:

   The second insert will fail due to duplicate key error -> Option C

4. Quick Check:

   Unique index blocks duplicates [OK]

Hint: Unique index stops duplicate values in that field [OK]

Common Mistakes:

• Assuming duplicates are allowed with unique index
• Thinking MongoDB auto-updates duplicates
• Believing index is ignored on duplicates

4. What is wrong with this command? db.products.createIndex({ price: 'ascending' })

medium

A. The field name must be a string

B. The collection name is missing

C. createIndex cannot be used on numeric fields

D. The index order must be 1 or -1, not 'ascending'

Solution

1. Step 1: Check index order values

   MongoDB requires 1 for ascending or -1 for descending, not strings like 'ascending'.

2. Step 2: Identify error cause

   Using 'ascending' causes syntax error because it's not a valid order value.

3. Final Answer:

   The index order must be 1 or -1, not 'ascending' -> Option D

4. Quick Check:

   Index order must be 1 or -1 [OK]

Hint: Use 1 or -1 for index order, not words [OK]

Common Mistakes:

• Using string values instead of 1 or -1
• Thinking createIndex can't index numbers
• Confusing collection name with index fields

5. You want to create a compound index on lastName (ascending) and age (descending) fields, ensuring no two documents have the same combination. Which command is correct?

hard

A. db.collection.createIndex({ lastName: -1, age: 1 }, { unique: false })

B. db.collection.createIndex({ lastName: 1, age: -1 }, { unique: true })

C. db.collection.createIndex({ lastName: 1, age: 1 }, { unique: true })

D. db.collection.createIndex({ lastName: 'asc', age: 'desc' }, { unique: true })

Solution

1. Step 1: Understand compound index syntax

   Compound index uses an object with multiple fields and their order: 1 for ascending, -1 for descending.

2. Step 2: Check uniqueness and order

   db.collection.createIndex({ lastName: 1, age: -1 }, { unique: true }) matches ascending lastName (1), descending age (-1), and unique true to prevent duplicates.

3. Step 3: Verify other options

   db.collection.createIndex({ lastName: -1, age: 1 }, { unique: false }) reverses order and is not unique; C uses ascending age instead of descending; D uses invalid string values.

4. Final Answer:

   db.collection.createIndex({ lastName: 1, age: -1 }, { unique: true }) -> Option B

5. Quick Check:

   Compound unique index uses correct order and unique:true [OK]

Hint: Use 1/-1 for order and unique:true for no duplicates [OK]

Common Mistakes:

• Using strings 'asc'/'desc' instead of 1/-1
• Setting unique:false when uniqueness is needed
• Mixing ascending and descending order incorrectly