Unique index behavior in MongoDB - Step-by-Step Execution

Concept Flow - Unique index behavior

Create Unique Index

↓

Insert Document

↓

Check if value exists in index

↓

Reject Insert

↓

Error: Duplicate

↓

End

When inserting a document, MongoDB checks the unique index for duplicates. If a duplicate exists, insertion is rejected with an error; otherwise, the document is added.

Execution Sample

MongoDB

db.users.createIndex({email: 1}, {unique: true});
db.users.insertOne({email: "[e-mail]"});
db.users.insertOne({email: "[e-mail]"});

Creates a unique index on 'email' and tries to insert two documents with the same email, showing the duplicate error on the second insert.

Execution Table

Step	Action	Index Check	Result	Database State
1	Create unique index on 'email'	N/A	Index created	Empty collection, index ready
2	Insert document {email: '[e-mail]'}	No duplicate found	Insert successful	Collection has 1 document with email '[e-mail]'
3	Insert document {email: '[e-mail]'}	Duplicate found	Insert rejected with duplicate key error	Collection unchanged with 1 document

💡 Insertion stops on duplicate key error at step 3

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
Collection documents	[]	[]	[{email: '[e-mail]'}]	[{email: '[e-mail]'}]
Unique index on email	None	Exists	Exists	Exists

Key Moments - 2 Insights

Why does the second insert fail even though it looks like a normal insert?

Because the unique index on 'email' prevents duplicate values. The execution_table row 3 shows the duplicate check fails and insertion is rejected.

What happens to the collection after a failed insert due to unique index?

The collection remains unchanged. As shown in execution_table row 3, the document count stays the same after the failed insert.

Visual Quiz - 3 Questions

Look at the execution_table at step 2. What does the index check find?

A No duplicate found

B Duplicate found

C Index does not exist

D Index is corrupted

💡 Hint

Refer to the 'Index Check' column at step 2 in execution_table

At which step does the insertion get rejected due to a duplicate key?

A Step 1

B Step 2

C Step 3

D No rejection occurs

💡 Hint

Check the 'Result' column in execution_table for the rejection message

If the unique index was removed before step 3, what would happen?

A First insert would fail

B Second insert would succeed

C Second insert would still fail

D Both inserts would fail

💡 Hint

Look at variable_tracker showing the presence of the unique index

Concept Snapshot

Unique index in MongoDB ensures no duplicate values in indexed fields.
When inserting, MongoDB checks the index for duplicates.
If duplicate found, insertion is rejected with an error.
If no duplicate, document is added to collection.
Unique indexes help maintain data integrity by preventing duplicates.

Full Transcript

This visual execution shows how MongoDB handles unique indexes. First, a unique index is created on the 'email' field. When inserting the first document with email '[e-mail]', MongoDB checks the index and finds no duplicates, so the insert succeeds. When inserting a second document with the same email, the index check finds a duplicate, so MongoDB rejects the insert with a duplicate key error. The collection remains unchanged after the failed insert. This process ensures that no two documents have the same value in the unique indexed field, maintaining data integrity.

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