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MongoDBquery~5 mins

Custom _id values in MongoDB

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Introduction

Every document in MongoDB needs a unique _id. Using custom _id values lets you control this unique identifier instead of letting MongoDB create one automatically.

When you want to use meaningful IDs like usernames or email addresses as the document ID.
When you need to link documents across collections using a known ID.
When importing data from another system that already has unique IDs.
When you want to avoid the extra space of MongoDB's default ObjectId.
When you want to quickly find documents by a known key without extra indexing.
Syntax
MongoDB
db.collection.insertOne({ _id: customValue, otherField: value })

The _id field can be any unique value: string, number, or object.

If you don't provide _id, MongoDB creates a unique ObjectId automatically.

Examples
Insert a user with a string _id "user123".
MongoDB
db.users.insertOne({ _id: "user123", name: "Alice" })
Insert a product with a numeric _id 1001.
MongoDB
db.products.insertOne({ _id: 1001, name: "Pen", price: 1.5 })
Insert an order with a custom object as _id.
MongoDB
db.orders.insertOne({ _id: { orderNumber: 500, region: "US" }, total: 250 })
Sample Program

This example inserts a product with a custom string _id "sku123" and then finds it using that _id.

MongoDB
use shop

// Insert a product with custom _id
 db.products.insertOne({ _id: "sku123", name: "Notebook", price: 3.99 })

// Find the product by _id
 db.products.find({ _id: "sku123" })
OutputSuccess
Important Notes

Custom _id values must be unique in the collection; duplicates cause errors.

Using meaningful _id values can make queries faster since _id is always indexed.

Summary

Every MongoDB document needs a unique _id.

You can set your own _id value instead of using the default ObjectId.

Custom _id helps with meaningful IDs and faster lookups.

Practice

(1/5)
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: "user@example.com", name: "User" }

// Option B
{ _id: "user@example.com", name: "User" }

// Option C
{ email: "user@example.com", name: "User" }

// Option D
{ _id: UUID(), email: "user@example.com", 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