insertOne method in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this insertOne operation?

Consider the following MongoDB insertOne command. What will be the output after running it?

MongoDB

db.users.insertOne({name: "Alice", age: 30})

A SyntaxError: missing parentheses in call to 'insertOne'

B {"acknowledged": false, "insertedId": null}

C {"acknowledged": true, "insertedId": ObjectId("some_id")}

D TypeError: insertOne is not a function

💡 Hint

insertOne returns an object with acknowledged true and the inserted document's id.

The insertOne method returns an object confirming the insertion with acknowledged set to true and the insertedId containing the new document's unique ObjectId.

📝 Syntax

intermediate

Which option has the correct syntax to insert a document with insertOne?

Choose the option that correctly uses insertOne to add a document with fields name and age.

A db.collection.insertOne(name: "Bob", age: 25)

B db.collection.insertOne({name: "Bob", age: 25})

C db.collection.insertOne(["name": "Bob", "age": 25])

D db.collection.insertOne({"name" = "Bob", "age" = 25})

💡 Hint

insertOne expects a single document object inside curly braces.

The correct syntax uses curly braces with key-value pairs separated by colons inside insertOne. Other options have syntax errors like missing braces or wrong separators.

❓ optimization

advanced

Which insertOne usage is best to ensure the document has a unique email field?

You want to insert a user document but avoid duplicates on the email field. Which option helps achieve this?

A db.users.insertOne({name: "Eve", email: "[e-mail]"})

B db.users.insertOne({name: "Eve", email: "[e-mail]"}, {unique: true})

C db.users.insertOne({name: "Eve", email: "[e-mail]"}, {upsert: true})

D db.users.createIndex({email: 1}, {unique: true}); db.users.insertOne({name: "Eve", email: "[e-mail]"})

💡 Hint

Unique constraints are set by indexes, not by insertOne options.

To ensure uniqueness on a field, create a unique index on that field before inserting. insertOne alone does not enforce uniqueness.

🔧 Debug

advanced

Why does this insertOne command fail with a TypeError?

Given the code: db.users.insertOne([{"name": "John"}]) Why does it raise a TypeError?

MongoDB

db.users.insertOne([{"name": "John"}])

A Because insertOne expects a single document object, not an array.

B Because the document is missing a required _id field.

C Because the collection 'users' does not exist.

D Because the name field must be a string, not an object.

💡 Hint

insertOne accepts one document, not an array of documents.

insertOne requires a single document object. Passing an array causes a TypeError because the method cannot handle arrays.

🧠 Conceptual

expert

What happens if you call insertOne with a document missing the _id field?

In MongoDB, if you run insertOne({name: "Sam"}) without specifying _id, what will the database do?

A MongoDB automatically generates a unique ObjectId for the _id field.

B The insertOne operation fails with a missing _id error.

C The document is inserted with _id set to null.

D MongoDB assigns the _id field the value of the name field.

💡 Hint

MongoDB always requires a unique _id for each document.

If _id is not provided, MongoDB creates a unique ObjectId automatically to ensure each document has a unique identifier.

Practice

1. What does the insertOne method do in MongoDB?

easy

A. Adds a single document to a collection

B. Deletes a document from a collection

C. Updates multiple documents in a collection

D. Finds documents matching a query

Solution

1. Step 1: Understand the purpose of insertOne

   The insertOne method is designed to add exactly one new document to a MongoDB collection.

2. Step 2: Compare with other operations

   Deleting, updating, or finding documents are different operations and use other methods like deleteOne, updateMany, or find.

3. Final Answer:

   Adds a single document to a collection -> Option A

4. Quick Check:

   insertOne = Adds one document [OK]

Hint: insertOne always adds exactly one document [OK]

Common Mistakes:

• Confusing insertOne with update or delete methods
• Thinking insertOne adds multiple documents
• Assuming insertOne returns the document itself

2. Which of the following is the correct syntax to insert a document with insertOne in MongoDB?

easy

A. db.collection.insertOne['name', 'Alice', 'age', 25]

B. db.collection.insertOne({name: 'Alice', age: 25})

C. db.collection.insertOne('name: Alice, age: 25')

D. db.collection.insertOne(name='Alice', age=25)

Solution

1. Step 1: Identify correct method call format

   The insertOne method requires a single document as an object inside parentheses.

2. Step 2: Check each option's syntax

   db.collection.insertOne({name: 'Alice', age: 25}) uses correct JavaScript object notation inside parentheses. Options B, C, and D use incorrect syntax for MongoDB commands.

3. Final Answer:

   db.collection.insertOne({name: 'Alice', age: 25}) -> Option B

4. Quick Check:

   insertOne uses object in parentheses [OK]

Hint: Use curly braces inside parentheses for insertOne [OK]

Common Mistakes:

• Using square brackets instead of parentheses
• Passing a string instead of an object
• Using assignment syntax inside insertOne

3. What will be the output of the following code?

const result = db.users.insertOne({username: 'john_doe', active: true});
printjson(result);

medium

A. An error because printjson is not defined

B. The inserted document itself

C. { acknowledged: true, insertedId: ObjectId('...') }

D. An empty object {}

Solution

1. Step 1: Understand insertOne return value

   The insertOne method returns an object with acknowledged true and the insertedId of the new document.

2. Step 2: Analyze printjson output

   Printing the result shows this object, not the document itself or an error.

3. Final Answer:

   { acknowledged: true, insertedId: ObjectId('...') } -> Option C

4. Quick Check:

   insertOne returns confirmation object [OK]

Hint: insertOne returns status and new document ID [OK]

Common Mistakes:

• Expecting the inserted document as output
• Thinking printjson causes an error
• Assuming insertOne returns empty object

4. Identify the error in this code snippet:

db.products.insertOne(name: 'Book', price: 15);

medium

A. Missing curly braces around the document

B. insertOne cannot insert documents with price field

C. Using semicolon instead of comma between fields

D. insertOne requires two arguments

Solution

1. Step 1: Check document syntax for insertOne

   The document to insert must be an object enclosed in curly braces {}.

2. Step 2: Identify the error in the code

   The code passes fields without curly braces, which is invalid syntax for insertOne.

3. Final Answer:

   Missing curly braces around the document -> Option A

4. Quick Check:

   insertOne needs object with braces [OK]

Hint: Always wrap inserted data in curly braces { } [OK]

Common Mistakes:

• Omitting curly braces for the document
• Using semicolons inside object literal
• Thinking insertOne needs multiple arguments

5. You want to insert a user document with fields name, email, and age. Which insertOne call correctly adds this document and returns the new document's ID?

hard

A. const res = db.users.insertOne('name: Eva, email: [e-mail], age: 30'); return res.insertedId;

B. const res = db.users.insertOne(['name', 'Eva', 'email', '[e-mail]', 'age', 30]); return res.insertedId;

C. const res = db.users.insertOne({name: 'Eva', email: '[e-mail]', age: 30}); return res;

D. const res = db.users.insertOne({name: 'Eva', email: '[e-mail]', age: 30}); return res.insertedId;

Solution

1. Step 1: Verify correct document format and method usage

   The document must be an object with keys and values inside curly braces. The insertOne method returns an object containing insertedId.

2. Step 2: Check return value for new document ID

   const res = db.users.insertOne({name: 'Eva', email: '[e-mail]', age: 30}); return res.insertedId; correctly returns res.insertedId, which is the new document's unique ID. const res = db.users.insertOne({name: 'Eva', email: '[e-mail]', age: 30}); return res; returns the whole result object, not just the ID. Options A and B use wrong argument types.

3. Final Answer:

   const res = db.users.insertOne({name: 'Eva', email: '[e-mail]', age: 30}); return res.insertedId; -> Option D

4. Quick Check:

   insertOne returns insertedId in result [OK]

Hint: insertOne returns insertedId inside result object [OK]

Common Mistakes:

• Passing array or string instead of object
• Returning whole result instead of insertedId
• Not wrapping fields in curly braces