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:<br>

{ _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.