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.