Custom _id values in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the default type of the _id field in MongoDB documents?

The default _id field in MongoDB is an ObjectId, a 12-byte unique identifier automatically generated for each document.

beginner

Can you assign a custom value to the _id field in MongoDB?

Yes, you can assign any unique value to the _id field when inserting a document, such as a string, number, or ObjectId, as long as it is unique within the collection.

intermediate

Why might you want to use a custom _id value instead of the default ObjectId?

Using a custom _id can help you use meaningful identifiers like usernames or codes, avoid extra lookups, or integrate with existing systems that have their own unique IDs.

beginner

What happens if you insert two documents with the same custom _id value?

MongoDB will reject the second insert with a duplicate key error because _id must be unique in a collection.

beginner

How do you insert a document with a custom _id in MongoDB using the shell?

Use the insert command with the _id field set to your custom value, for example:

{ _id: "user123", name: "Alice" }

What type of value can the _id field have in MongoDB?

A Only ObjectId

B Any unique value

C Only strings

D Only numbers

The _id field can be any unique value, not just ObjectId.

What happens if you insert a document without specifying _id?

A MongoDB automatically creates an ObjectId for <code>_id</code>

B MongoDB rejects the insert

C The document has no <code>_id</code>

D You must specify <code>_id</code> manually

MongoDB automatically generates an ObjectId for _id if you don't specify one.

Why is the _id field important in MongoDB?

A It stores the document's creation date

B It is optional and has no special role

C It uniquely identifies each document

D It stores the document's size

_id uniquely identifies each document in a collection.

If you want to use a username as the _id, what must you ensure?

A Usernames are unique in the collection

B Usernames are always numbers

C Usernames are ObjectIds

D Usernames are optional

The _id must be unique, so usernames used as _id must be unique.

What error occurs if you insert a document with a duplicate _id?

A Syntax error

B No error, it overwrites

C Connection error

D Duplicate key error

MongoDB throws a duplicate key error if _id is not unique.

Explain how and why you might use a custom _id value in MongoDB.

Think about identifiers you already use in real life, like usernames or codes.

Describe what happens if you try to insert two documents with the same _id in MongoDB.

Consider the role of <code>_id</code> as a unique identifier.

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