Unique index behavior in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

MongoDB

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

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

Atrue

Bfalse

Cunique

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of a boolean for the unique option.

Setting unique to false which does not enforce uniqueness.

Setting unique: true ensures the index enforces uniqueness on the field.

2fill in blank

medium

Complete the code to insert a document that violates a unique index on "username".

MongoDB

db.users.insertOne({ username: [1], age: 25 })

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

A"unique"

Bjohn_doe

C25

D"john_doe"

Attempts:

3 left

💡 Hint

Common Mistakes

Not quoting string values causing syntax errors.

Using a number instead of a string for username.

String values must be quoted in MongoDB insert commands.

3fill in blank

hard

Fix the error in the unique index creation by completing the code.

MongoDB

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

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

AuniqueIndex

Bunique

Cunique_key

Dunique_value

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect option names like uniqueIndex or unique_key.

Misspelling the option name.

The correct option name to enforce uniqueness is unique.

4fill in blank

hard

Fill both blanks to create a unique index on "phone" field in descending order.

MongoDB

db.contacts.createIndex({ phone: [1] }, { [2]: true })

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

A-1

B1

Cunique

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of -1 for descending order.

Using index instead of unique for uniqueness.

Use -1 for descending order and unique: true to enforce uniqueness.

5fill in blank

hard

Fill all three blanks to create a unique compound index on "firstName" ascending and "lastName" descending.

MongoDB

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

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

A1

B-1

Cunique

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up ascending and descending values.

Using index instead of unique for uniqueness.

Use 1 for ascending, -1 for descending, and unique: true to enforce uniqueness on the compound index.

Practice

1. What is the main purpose of a unique index in MongoDB?

easy

A. To ensure no two documents have the same value for the indexed field

B. To speed up queries without any restrictions on data

C. To allow duplicate values but sort them quickly

D. To store data in a compressed format

Solution

1. Step 1: Understand unique index function

   A unique index prevents duplicate values in the indexed field across documents.

2. Step 2: Compare options

   Only To ensure no two documents have the same value for the indexed field describes this behavior correctly; others describe unrelated features.

3. Final Answer:

   To ensure no two documents have the same value for the indexed field -> Option A

4. Quick Check:

   Unique index = no duplicates [OK]

Hint: Unique index means no duplicate values allowed [OK]

Common Mistakes:

• Thinking unique index speeds queries without restrictions
• Confusing unique index with data compression
• Believing unique index allows duplicates

2. Which of the following is the correct way to create a unique index on the field email in MongoDB?

easy

A. db.collection.createUniqueIndex({ email: 1 })

B. db.collection.createIndex({ email: 1 }, { duplicate: false })

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

D. db.collection.ensureIndex({ email: 1 }, { unique: false })

Solution

1. Step 1: Recall MongoDB syntax for unique index

   The correct syntax uses createIndex with { unique: true } option.

2. Step 2: Evaluate options

   db.collection.createIndex({ email: 1 }, { unique: true }) matches the correct syntax. db.collection.createIndex({ email: 1 }, { duplicate: false }) uses invalid option duplicate. db.collection.createUniqueIndex({ email: 1 }) uses a non-existent method. db.collection.ensureIndex({ email: 1 }, { unique: false }) sets unique to false, which is incorrect.

3. Final Answer:

   db.collection.createIndex({ email: 1 }, { unique: true }) -> Option C

4. Quick Check:

   Use createIndex with unique:true [OK]

Hint: Use createIndex with { unique: true } to enforce uniqueness [OK]

Common Mistakes:

• Using wrong option name like 'duplicate'
• Using non-existent methods like createUniqueIndex
• Setting unique to false by mistake

3. Given a collection with a unique index on username, what happens if you run this insert twice?

db.users.insertOne({ username: "alice" })

medium

A. Both inserts succeed, creating two identical documents

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

C. The second insert overwrites the first document

D. MongoDB ignores the unique index and inserts both

Solution

1. Step 1: Understand unique index effect on inserts

   A unique index on username prevents duplicate values in that field.

2. Step 2: Predict behavior on duplicate insert

   The first insert succeeds. The second insert with the same username causes a duplicate key error and fails.

3. Final Answer:

   The second insert will fail with a duplicate key error -> Option B

4. Quick Check:

   Duplicate insert with unique index = error [OK]

Hint: Duplicate insert on unique field causes error [OK]

Common Mistakes:

• Assuming duplicates are allowed
• Thinking second insert overwrites first
• Believing MongoDB ignores unique index

4. You created a unique index on email but still see duplicate emails in your collection. What is the most likely cause?

medium

A. The unique index was created after duplicates already existed

B. The unique index was created with the wrong field name

C. MongoDB does not enforce uniqueness on indexes

D. The collection is empty

Solution

1. Step 1: Understand unique index creation behavior

   If duplicates exist before creating a unique index, MongoDB will fail to create it unless duplicates are removed or ignored.

2. Step 2: Analyze why duplicates remain

   If the index was forced or created without validation, duplicates remain. The unique index was created after duplicates already existed explains this scenario. The unique index was created with the wrong field name would cause index creation failure. MongoDB does not enforce uniqueness on indexes is false. The collection is empty is irrelevant.

3. Final Answer:

   The unique index was created after duplicates already existed -> Option A

4. Quick Check:

   Existing duplicates block unique index enforcement [OK]

Hint: Create unique index before duplicates exist [OK]

Common Mistakes:

• Assuming MongoDB ignores duplicates silently
• Not checking field name correctness
• Thinking empty collection causes duplicates

5. You want to ensure uniqueness on user.email in documents where user.active is true only. How can you create a unique index for this condition?

hard

A. Create a compound unique index on user.email and user.active without filter

B. Create a unique index on user.email without any filter

C. Create a unique index on user.active only

D. Create a partial unique index with filter { 'user.active': true } on field user.email

Solution

1. Step 1: Understand partial unique index

   A partial unique index applies uniqueness only to documents matching a filter condition.

2. Step 2: Apply partial index to user.email where user.active is true

   Create a partial unique index with filter { 'user.active': true } on field user.email correctly creates a partial unique index on user.email with filter { 'user.active': true }. Create a unique index on user.email without any filter enforces uniqueness on all documents. Create a compound unique index on user.email and user.active without filter enforces uniqueness on combined fields but not conditional. Create a unique index on user.active only is unrelated.

3. Final Answer:

   Create a partial unique index with filter { 'user.active': true } on field user.email -> Option D

4. Quick Check:

   Partial unique index = conditional uniqueness [OK]

Hint: Use partial unique index with filter for conditional uniqueness [OK]

Common Mistakes:

• Creating unique index without filter for conditional uniqueness
• Using compound index without filter
• Indexing wrong field only