Unique index behavior in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a unique index in MongoDB?

A unique index in MongoDB ensures that the indexed field does not have duplicate values across documents in a collection. It helps keep data unique and prevents duplicate entries.

beginner

What happens if you try to insert a document with a duplicate value on a unique indexed field?

MongoDB will reject the insert operation and return an error because the unique index prevents duplicate values in that field.

intermediate

Can a unique index be created on multiple fields in MongoDB?

Yes, MongoDB supports unique compound indexes that enforce uniqueness across the combination of multiple fields together, not just one field.

intermediate

How does MongoDB treat documents with missing fields when a unique index is created on that field?

MongoDB allows multiple documents to have missing or null values for a unique indexed field because null is treated as a unique value for each document.

beginner

How can you create a unique index on the 'email' field in a MongoDB collection named 'users'?

Use the command: db.users.createIndex({ email: 1 }, { unique: true }) to create a unique index on the 'email' field.

What does a unique index in MongoDB prevent?

A Indexes on multiple fields

B Null values in the indexed field

C All documents from being inserted

D Duplicate values in the indexed field

A unique index prevents duplicate values in the indexed field, ensuring each value is unique.

What happens if you insert a document with a duplicate value on a unique indexed field?

A MongoDB returns an error and rejects the insert

B The duplicate is ignored silently

C MongoDB updates the existing document

D The insert succeeds without error

MongoDB rejects the insert and returns an error to maintain uniqueness.

Can a unique index be created on multiple fields in MongoDB?

A No, only single fields can have unique indexes

B Yes, using a compound unique index

C Yes, but only if fields are numeric

D No, MongoDB does not support unique indexes

MongoDB supports compound unique indexes that enforce uniqueness across multiple fields combined.

How does MongoDB treat multiple documents missing a field with a unique index on that field?

A It rejects all but one document

B It treats missing fields as duplicates

C It allows multiple documents with missing or null values

D It converts missing fields to empty strings

MongoDB allows multiple documents to have missing or null values for a unique indexed field.

Which command creates a unique index on the 'username' field in the 'accounts' collection?

A db.accounts.createIndex({ username: 1 }, { unique: true })

B db.accounts.createIndex({ username: 1 })

C db.accounts.ensureIndex({ username: 1 }, true)

D db.accounts.createUniqueIndex({ username: 1 })

The correct syntax to create a unique index is createIndex with the option { unique: true }.

Explain what a unique index is in MongoDB and how it affects data insertion.

Think about how MongoDB keeps certain fields unique across documents.

Describe how MongoDB handles documents with missing or null values when a unique index is applied on that field.

Consider how null values are treated differently than duplicates.

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