Auto-generated _id behavior in MongoDB

Introduction

MongoDB automatically creates a unique _id for each document to identify it easily without extra work.

When you insert a new document and don't provide an _id, MongoDB creates one for you.

When you want to ensure each document has a unique identifier without manually generating it.

When you want to quickly find or update a document by its unique _id.

When you want to avoid duplicate documents in a collection.

When you want to link documents using their unique _id values.

Syntax

MongoDB

db.collection.insertOne({ field1: value1, field2: value2 })

If you do not specify the _id field, MongoDB adds it automatically.

The auto-generated _id is an ObjectId, which is a 12-byte unique value.

Examples

Inserts a new user document. MongoDB creates a unique _id automatically.

MongoDB

db.users.insertOne({ name: "Alice", age: 30 })

Inserts a user with a custom _id instead of auto-generated one.

MongoDB

db.users.insertOne({ _id: "customID123", name: "Bob" })

Finds a document by its auto-generated ObjectId _id.

MongoDB

db.users.findOne({ _id: ObjectId("507f1f77bcf86cd799439011") })

Sample Program

This inserts a product without specifying _id. MongoDB creates a unique _id automatically. Then it finds and shows the inserted document using that _id.

MongoDB

use testdb

// Insert a document without _id
var result = db.products.insertOne({ name: "Pen", price: 1.5 })

// Show the inserted document with auto-generated _id
var doc = db.products.findOne({ _id: result.insertedId })
doc

Important Notes

The ObjectId contains a timestamp, machine id, process id, and a counter to ensure uniqueness.

You can provide your own _id, but it must be unique in the collection.

Trying to insert a document with a duplicate _id will cause an error.

Summary

MongoDB auto-generates a unique _id for each document if you don't provide one.

The _id is an ObjectId by default, which helps uniquely identify documents.

You can use the _id to quickly find, update, or delete documents.

Practice

1. In MongoDB, what happens if you insert a document without specifying the _id field?

easy

A. MongoDB automatically generates a unique _id for the document.

B. The insert operation fails with an error.

C. The document is inserted with a null _id.

D. MongoDB assigns a sequential integer as the _id.

Solution

1. Step 1: Understand MongoDB's default behavior for _id

   MongoDB requires each document to have a unique _id. If not provided, it creates one automatically.

2. Step 2: Identify the type of auto-generated _id

   The auto-generated _id is an ObjectId, which is unique and generated by MongoDB.

3. Final Answer:

   MongoDB automatically generates a unique _id for the document. -> Option A

4. Quick Check:

   Auto-generated _id = unique ObjectId [OK]

Hint: If no _id, MongoDB creates a unique one automatically [OK]

Common Mistakes:

• Thinking insert fails without _id
• Assuming _id can be null
• Believing _id is a simple number

2. Which of the following is the correct way to insert a document without specifying _id in MongoDB shell?

easy

A. db.collection.insertOne({_id: 1, name: 'Alice'})

B. db.collection.insertOne({_id: null, name: 'Alice'})

C. db.collection.insertOne({name: 'Alice'})

D. db.collection.insertOne()

Solution

1. Step 1: Check syntax for inserting a document without _id

   The correct syntax is to provide the document fields except _id, so MongoDB generates it.

2. Step 2: Evaluate each option

   db.collection.insertOne({name: 'Alice'}) inserts a document with only the name field, letting MongoDB create _id. db.collection.insertOne({_id: null, name: 'Alice'}) sets _id to null which is invalid. db.collection.insertOne({_id: 1, name: 'Alice'}) sets _id manually. db.collection.insertOne() is missing the document argument.

3. Final Answer:

   db.collection.insertOne({name: 'Alice'}) -> Option C

4. Quick Check:

   Insert without _id uses document only [OK]

Hint: Insert document without _id to auto-generate it [OK]

Common Mistakes:

• Passing empty insertOne() without document
• Setting _id to null explicitly
• Confusing manual and automatic _id assignment

3. Consider the following MongoDB shell commands:

db.test.insertOne({name: 'Bob'})
db.test.insertOne({_id: ObjectId('507f1f77bcf86cd799439011'), name: 'Carol'})
db.test.find().count()

What will be the output of the count() command?

medium

A. 2

B. 0

C. 1

D. Error due to duplicate _id

Solution

1. Step 1: Analyze the inserts

   The first insert adds a document without _id, so MongoDB generates one. The second insert adds a document with a specific _id ObjectId.

2. Step 2: Check for duplicates and count documents

   Since the _id in the second insert is unique and different from the first, both inserts succeed. So, the collection has 2 documents.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   Two unique documents inserted = count 2 [OK]

Hint: Unique _id means both inserts succeed [OK]

Common Mistakes:

• Assuming auto-generated _id matches manual one
• Thinking duplicate _id error occurs
• Forgetting count() returns total documents

4. You run this code in MongoDB shell:

db.users.insertOne({_id: 1, name: 'Dave'})
db.users.insertOne({_id: 1, name: 'Eve'})

What will happen and how can you fix it?

medium

A. Second insert overwrites the first document silently.

B. Both inserts succeed; MongoDB allows duplicate _id.

C. First insert fails; _id must be ObjectId.

D. Second insert fails due to duplicate _id; fix by using unique _id values.

Solution

1. Step 1: Understand _id uniqueness constraint

   MongoDB requires _id to be unique in a collection. Duplicate _id values cause insert failure.

2. Step 2: Analyze the inserts

   The first insert with _id: 1 succeeds. The second insert tries the same _id, causing a duplicate key error.

3. Final Answer:

   Second insert fails due to duplicate _id; fix by using unique _id values. -> Option D

4. Quick Check:

   Duplicate _id causes insert failure [OK]

Hint: Each _id must be unique to avoid insert errors [OK]

Common Mistakes:

• Thinking MongoDB allows duplicate _id
• Assuming _id must be ObjectId type
• Believing second insert overwrites first

5. You want to insert multiple documents into a MongoDB collection, but ensure each document has a unique _id without manually specifying it. Which approach correctly achieves this and why?

const docs = [
  {name: 'Anna'},
  {name: 'Ben'},
  {name: 'Cara'}
];
db.collection.insertMany(docs);

hard

A. Insert documents without _id but create a unique index on name.

B. Insert documents as is; MongoDB auto-generates unique _id for each document.

C. Manually assign sequential integers as _id before insert.

D. Add _id: null to each document to let MongoDB generate _id.

Solution

1. Step 1: Understand MongoDB's auto-generation of _id

   When documents lack _id, MongoDB automatically creates a unique ObjectId for each during insert.

2. Step 2: Evaluate each option

   Insert documents as is; MongoDB auto-generates unique _id for each document. correctly relies on MongoDB's default behavior. Add _id: null to each document to let MongoDB generate _id. is invalid because _id: null is not allowed. Manually assign sequential integers as _id before insert. requires manual work and risks duplicates. Insert documents without _id but create a unique index on name. creates a unique index on name, unrelated to _id uniqueness.

3. Final Answer:

   Insert documents as is; MongoDB auto-generates unique _id for each document. -> Option B

4. Quick Check:

   Missing _id means MongoDB creates unique ObjectId [OK]

Hint: Insert without _id to get unique ObjectId automatically [OK]

Common Mistakes:

• Setting _id to null explicitly
• Manually assigning _id unnecessarily
• Confusing unique index on other fields with _id