Custom _id values in MongoDB - Step-by-Step Execution

Concept Flow - Custom _id values

Start Insert Document

↓

Check if _id provided?

No→Generate default ObjectId

Yes↓

Use provided _id value

↓

Insert document with _id

↓

Document stored in collection

When inserting a document, MongoDB checks if you provide an _id. If yes, it uses that value; if no, it creates a default ObjectId.

Execution Sample

MongoDB

db.users.insertOne({_id: "user123", name: "Alice"})

Insert a document with a custom _id value 'user123' into the users collection.

Execution Table

Step	Action	Check _id	Resulting _id	Insert Status
1	Start insert document	N/A	N/A	Pending
2	Check if _id provided	Yes (_id = 'user123')	Use 'user123'	Pending
3	Insert document with custom _id	N/A	user123	Success
4	Document stored in collection	N/A	user123	Complete

💡 Insertion completes using the provided custom _id 'user123'.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
_id	undefined	"user123"	"user123"	"user123"
Insert Status	Not started	Pending	Success	Complete

Key Moments - 2 Insights

What happens if I do not provide an _id when inserting?

If no _id is provided (see execution_table step 2), MongoDB automatically generates a unique ObjectId for the document.

Can I use any value as a custom _id?

Yes, you can use any unique value as _id (string, number, etc.), but it must be unique in the collection to avoid insertion errors.

Visual Quiz - 3 Questions

Look at the execution_table, what _id value is used at step 3?

A null

B "user123"

C A generated ObjectId

D undefined

💡 Hint

Check the 'Resulting _id' column at step 3 in the execution_table.

At which step does MongoDB decide to use the provided _id?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Check _id' column to see when the _id presence is checked.

If you omit _id in the insert, what changes in the execution_table?

A Step 3 fails with error

B Step 4 is skipped

C Step 2 shows 'No' and a generated ObjectId is used

D Nothing changes

💡 Hint

Think about the flow when _id is not provided as shown in concept_flow.

Concept Snapshot

Insert documents with custom _id by specifying _id field.
If _id is missing, MongoDB creates a unique ObjectId.
Custom _id must be unique in the collection.
Use any data type for _id (string, number, etc.).
Insertion uses provided _id directly without modification.

Full Transcript

When you insert a document into MongoDB, it checks if you gave an _id value. If you did, it uses that as the document's unique identifier. If you didn't, MongoDB creates a unique ObjectId automatically. This _id is important because it uniquely identifies each document in the collection. You can use any value for _id, like a string or number, but it must be unique to avoid errors. The insertion process stores the document with the chosen _id in the collection.

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