findOne method in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find one document in the 'users' collection.

MongoDB

db.users.[1]({})

Drag options to blanks, or click blank then click option'

AupdateOne

BfindOne

CinsertOne

Dfind

Attempts:

3 left

💡 Hint

Common Mistakes

Using find instead of findOne returns a cursor, not a single document.

The findOne method returns a single document from the collection that matches the query.

2fill in blank

medium

Complete the code to find one user with the name 'Alice'.

MongoDB

db.users.findOne({ name: [1] })

Drag options to blanks, or click blank then click option'

A'David'

B'Bob'

C'Alice'

D'Charlie'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string value.

Using the wrong name.

The query object specifies name: 'Alice' to find a user named Alice.

3fill in blank

hard

Fix the error in the code to find one document where age is 30.

MongoDB

db.users.findOne({ age: [1] })

Drag options to blanks, or click blank then click option'

A30

B"30"

C'30'

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around numbers, which makes them strings.

Using a variable name instead of a value.

The age value should be a number, not a string, so use 30 without quotes.

4fill in blank

hard

Fill both blanks to find one document where status is 'active' and return only the name field.

MongoDB

db.users.findOne({ status: [1] }, { [2]: 1 })

Drag options to blanks, or click blank then click option'

A'active'

Bstatus

Cname

D'inactive'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong field name in the projection.

Using 'inactive' instead of 'active' in the query.

The query filters for status: 'active' and the projection returns only the name field.

5fill in blank

hard

Fill all three blanks to find one document where age is greater than 25 and return only the name and age fields.

MongoDB

db.users.findOne({ age: { [1]: [2] } }, { [3]: 1, age: 1 })

Drag options to blanks, or click blank then click option'

A$gt

B25

Cname

D$lt

Attempts:

3 left

💡 Hint

Common Mistakes

Using '$lt' instead of '$gt' for greater than.

Forgetting to include the 'name' field in the projection.

The query uses $gt: 25 to find ages greater than 25, and the projection includes name and age fields.

Practice

1. What does the findOne method do in MongoDB?

easy

A. It returns the first document that matches the filter criteria.

B. It returns all documents in the collection.

C. It deletes a document from the collection.

D. It updates a document in the collection.

Solution

1. Step 1: Understand the purpose of findOne

   The findOne method is designed to find a single document that matches the filter criteria in a MongoDB collection.

2. Step 2: Compare with other operations

   Unlike methods that return multiple documents or modify data, findOne only retrieves one matching document without changing the data.

3. Final Answer:

   It returns the first document that matches the filter criteria. -> Option A

4. Quick Check:

   findOne = single matching document [OK]

Hint: Remember: findOne returns only one matching document [OK]

Common Mistakes:

• Thinking findOne returns all documents
• Confusing findOne with update or delete methods
• Expecting findOne to modify data

2. Which of the following is the correct syntax to find one document with name equal to 'Alice' using findOne?

easy

A. db.collection.findOne({name: 'Alice'})

B. db.collection.findOne('name' = 'Alice')

C. db.collection.findOne(name == 'Alice')

D. db.collection.findOne({name == 'Alice'})

Solution

1. Step 1: Check the correct filter syntax

   In MongoDB, filters are passed as objects with key-value pairs, like {name: 'Alice'}.

2. Step 2: Validate the method call

   The correct syntax is db.collection.findOne({name: 'Alice'}). Other options use invalid operators or syntax.

3. Final Answer:

   db.collection.findOne({name: 'Alice'}) -> Option A

4. Quick Check:

   Filter object syntax = db.collection.findOne({name: 'Alice'}) [OK]

Hint: Use curly braces with key:value pairs for filters [OK]

Common Mistakes:

• Using '=' or '==' inside filter object
• Passing filter as a string
• Missing curly braces around filter

3. Given the collection users with documents:
{name: 'Bob', age: 30}, {name: 'Alice', age: 25}, {name: 'Bob', age: 22}
What will db.users.findOne({name: 'Bob'}) return?

medium

A. {name: 'Bob', age: 22}

B. {name: 'Alice', age: 25}

C. {name: 'Bob', age: 30}

D. null

Solution

1. Step 1: Understand findOne returns first match

   The findOne method returns the first document matching the filter in the collection's natural order.

2. Step 2: Identify the first matching document

   Documents are stored in insertion order. The first document with name: 'Bob' is {name: 'Bob', age: 30}.

3. Final Answer:

   {name: 'Bob', age: 30} -> Option C

4. Quick Check:

   First matching document = {name: 'Bob', age: 30} [OK]

Hint: findOne returns the first matching document found [OK]

Common Mistakes:

• Assuming findOne returns the last matching document
• Expecting all matches instead of one
• Confusing document order

4. What is wrong with this query?
db.users.findOne(name: 'Alice')

medium

A. The filter key should be capitalized.

B. Missing curly braces around the filter object.

C. The collection name is incorrect.

D. Using findOne instead of find.

Solution

1. Step 1: Check filter syntax in findOne

   The filter argument must be an object enclosed in curly braces, like {name: 'Alice'}.

2. Step 2: Identify the syntax error

   The query misses curly braces around the filter, causing a syntax error.

3. Final Answer:

   Missing curly braces around the filter object. -> Option B

4. Quick Check:

   Filter must be an object = Missing curly braces around the filter object. [OK]

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

Common Mistakes:

• Omitting curly braces for filter
• Confusing findOne with find
• Assuming key names are case sensitive

5. You want to find a user document with email '[e-mail]' but only want to return the name and age fields. Which findOne query is correct?

hard

A. db.users.findOne({email: '[e-mail]'}, {name: 1, age: 1, email: 0})

B. db.users.findOne({email: '[e-mail]'}, {email: 1})

C. db.users.findOne({email: '[e-mail]'}, {name: 1, age: 1})

D. db.users.findOne({email: '[e-mail]'}, {name: 1, age: 1, _id: 0})

Solution

1. Step 1: Understand projection in findOne

   The second argument to findOne is the projection object that specifies which fields to include (1) or exclude (0).

2. Step 2: Choose correct projection

   To return only name and age and exclude _id, use {name: 1, age: 1, _id: 0}. db.users.findOne({email: '[e-mail]'}, {name: 1, age: 1, _id: 0}) matches this.

3. Final Answer:

   db.users.findOne({email: '[e-mail]'}, {name: 1, age: 1, _id: 0}) -> Option D

4. Quick Check:

   Projection includes name and age only = db.users.findOne({email: '[e-mail]'}, {name: 1, age: 1, _id: 0}) [OK]

Hint: Use projection object with 1 to include fields, 0 to exclude [OK]

Common Mistakes:

• Forgetting to exclude _id when not needed
• Including unwanted fields in projection
• Using projection incorrectly as filter