Custom _id values in MongoDB - Mini Project: Build & Apply

Custom _id values in MongoDB

📖 Scenario: You are managing a small library database. Each book needs a unique identifier that you assign yourself instead of letting MongoDB create one automatically.

🎯 Goal: Create a MongoDB collection with books where each book has a custom _id value. Insert multiple books with your chosen _id values.

📋 What You'll Learn

Create a collection named books

Insert documents with a custom _id field

Each document must have _id, title, and author fields

Use string values for the custom _id fields

💡 Why This Matters

🌍 Real World

Custom _id values are useful when you want to control the unique identifiers in your database, such as using ISBNs for books or employee IDs.

💼 Career

Many jobs require managing databases where custom IDs help integrate with other systems or maintain meaningful keys.

1

Create the books collection with one book

Create a collection called books and insert one document with _id set to "book1", title set to "The Great Gatsby", and author set to "F. Scott Fitzgerald".

MongoDB

// Insert one book document with custom _id
// Your code here

Hint

Use db.books.insertOne() with a document that includes _id, title, and author.

2

Add a variable for the second book's _id

Create a variable called secondBookId and set it to the string "book2".

MongoDB

db.books.insertOne({_id: "book1", title: "The Great Gatsby", author: "F. Scott Fitzgerald"})
// Create variable secondBookId
// Your code here

Hint

Use const secondBookId = "book2" to store the custom ID for the next book.

3

Insert the second book using the secondBookId variable

Insert a new document into the books collection with _id set to the variable secondBookId, title set to "1984", and author set to "George Orwell".

MongoDB

db.books.insertOne({_id: "book1", title: "The Great Gatsby", author: "F. Scott Fitzgerald"})
const secondBookId = "book2"
// Insert second book using secondBookId
// Your code here

Hint

Use db.books.insertOne() with _id: secondBookId and the other fields.

4

Insert a third book with a custom _id directly

Insert a third document into the books collection with _id set to "book3", title set to "To Kill a Mockingbird", and author set to "Harper Lee".

MongoDB

db.books.insertOne({_id: "book1", title: "The Great Gatsby", author: "F. Scott Fitzgerald"})
const secondBookId = "book2"
db.books.insertOne({_id: secondBookId, title: "1984", author: "George Orwell"})
// Insert third book with _id "book3"
// Your code here

Hint

Use db.books.insertOne() with the specified fields and _id value.

Practice

1.

What is the purpose of the _id field in a MongoDB document?

easy

A. It uniquely identifies each document in a collection.

B. It stores the creation date of the document.

C. It holds the user's login information.

D. It contains the document's size in bytes.

Solution

1. Step 1: Understand the role of _id in MongoDB

   The _id field is a unique identifier for each document in a collection, ensuring no two documents share the same _id.

2. Step 2: Compare with other options

   Other options describe unrelated fields or metadata, not the unique identifier role.

3. Final Answer:

   It uniquely identifies each document in a collection. -> Option A

4. Quick Check:

   _id = unique document ID [OK]

Hint: Remember: _id means unique ID for each document [OK]

Common Mistakes:

• Thinking _id stores creation date
• Confusing _id with user data fields
• Assuming _id is optional

2.

Which of the following is the correct way to insert a document with a custom _id value in MongoDB?

db.users.insertOne({ _id: 123, name: "Alice" })

easy

A. db.users.insertOne({ id: 123, name: "Alice" })

B. db.users.insertOne({ _id: 123, name: "Alice" })

C. db.users.insertOne({ _id: "name", name: "Alice" })

D. db.users.insertOne({ _id: ObjectId(), name: "Alice" })

Solution

1. Step 1: Identify correct _id field usage

   The _id field must be named exactly _id to set a custom ID. db.users.insertOne({ _id: 123, name: "Alice" }) uses _id: 123 correctly.

2. Step 2: Check other options for errors

   db.users.insertOne({ id: 123, name: "Alice" }) uses id instead of _id. db.users.insertOne({ _id: "name", name: "Alice" }) uses a string "name" which is valid but less meaningful here. db.users.insertOne({ _id: ObjectId(), name: "Alice" }) uses default ObjectId, not custom.

3. Final Answer:

   db.users.insertOne({ _id: 123, name: "Alice" }) -> Option B

4. Quick Check:

   Custom _id needs exact field name [OK]

Hint: Use exact field name _id for custom IDs [OK]

Common Mistakes:

• Using id instead of _id
• Confusing ObjectId() with custom values
• Using invalid types for _id

3.

Given the following documents inserted into a collection:

<pre>db.products.insertMany([ { _id: "p1", name: "Pen" }, { _id: "p2", name: "Pencil" }, { _id: "p3", name: "Eraser" } ]) What will db.products.find({ _id: "p2" }).toArray() return?

medium

A. [{ _id: "p2", name: "Pencil" }]

B. [{ _id: "p1", name: "Pen" }]

C. []

D. Error: Invalid query

Solution

1. Step 1: Understand the query filter

   The query searches for a document with _id equal to "p2".

2. Step 2: Match the document in the collection

   The document with _id: "p2" has the name "Pencil" and exists in the collection.

3. Final Answer:

   [{ _id: "p2", name: "Pencil" }] -> Option A

4. Quick Check:

   Query by custom _id returns matching document [OK]

Hint: Query by exact _id returns matching document [OK]

Common Mistakes:

• Expecting multiple documents returned
• Confusing _id with other fields
• Assuming query returns error for string _id

4.

Consider this insertion attempt:

db.orders.insertOne({ _id: 101, item: "Book" })
db.orders.insertOne({ _id: 101, item: "Notebook" })

What error will occur and why?

medium

A. SyntaxError due to missing quotes around _id value.

B. TypeError because _id must be a string.

C. No error; both documents inserted successfully.

D. DuplicateKeyError because _id must be unique.

Solution

1. Step 1: Check uniqueness requirement of _id

   The _id field must be unique in a collection. Both documents use _id: 101.

2. Step 2: Identify the error caused by duplicate _id

   Inserting the second document with the same _id causes a DuplicateKeyError.

3. Final Answer:

   DuplicateKeyError because _id must be unique. -> Option D

4. Quick Check:

   Duplicate _id causes insertion error [OK]

Hint: No two documents can share the same _id [OK]

Common Mistakes:

• Thinking _id can repeat
• Confusing syntax error with duplicate key error
• Assuming _id must be string only

5.

You want to store user profiles with custom _id values based on their email addresses to speed up lookups. Which approach is best?

// Option A
{ _id: ObjectId(), email: "[e-mail]", name: "User" }

// Option B
{ _id: "[e-mail]", name: "User" }

// Option C
{ email: "[e-mail]", name: "User" }

// Option D
{ _id: UUID(), email: "[e-mail]", name: "User" }

hard

A. Use default ObjectId and store email separately.

B. Do not use _id, just store email field.

C. Set _id to the email string for direct lookup.

D. Use UUID as _id and email separately.

Solution

1. Step 1: Understand the goal of custom _id

   The goal is to speed up lookups by using email as the unique identifier.

2. Step 2: Evaluate options for best fit

   Setting _id to the email string enables direct lookup using the email as _id, making queries fast. Using default ObjectId or UUID with separate email requires additional indexing. Omitting _id is invalid since it is mandatory.

3. Final Answer:

   Set _id to the email string for direct lookup. -> Option C

4. Quick Check:

   Custom _id as email speeds queries [OK]

Hint: Use email as _id for fast direct lookups [OK]

Common Mistakes:

• Ignoring _id uniqueness requirement
• Using default IDs when custom IDs help
• Not indexing email for fast queries