find method basics in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with a specific field value

Given a collection users with documents containing age and name, what does the following query return?

db.users.find({ age: 25 })

MongoDB

db.users.find({ age: 25 })

A An error because the query syntax is invalid

B All documents where the age field is greater than 25

C All documents where the name field is '25'

D All documents where the age field is exactly 25

💡 Hint

Think about what the object inside find() means.

The find method with { age: 25 } returns all documents where the age field equals 25 exactly.

📝 Syntax

intermediate

Identify the syntax error in a find query

Which option contains a syntax error in the MongoDB find method?

MongoDB

db.products.find({ price: { $gt: 100 } })

A db.products.find({ price: { $gt: 100 } })

B db.products.find({ price: $gt: 100 })

C db.products.find({ price: { $lt: 50 } })

D db.products.find({ price: { $eq: 75 } })

💡 Hint

Look for missing or misplaced braces and colons.

Option B is missing braces around the $gt operator, causing a syntax error.

❓ query_result

advanced

Find documents with multiple conditions

What documents does this query return?

db.orders.find({ status: 'shipped', quantity: { $gte: 10 } })

MongoDB

db.orders.find({ status: 'shipped', quantity: { $gte: 10 } })

A Documents where quantity is at least 10 only

B Documents where status is 'shipped' or quantity is at least 10

C Documents where status is 'shipped' and quantity is at least 10

D Documents where status is 'shipped' and quantity is exactly 10

💡 Hint

Multiple fields in the query object mean AND condition.

MongoDB find with multiple fields matches documents where all conditions are true (AND).

🔧 Debug

advanced

Why does this find query return no results?

A collection employees has documents with a field department. The query below returns no documents, but you expect some.

db.employees.find({ department: 'Sales' })

What is the most likely reason?

A The field name is case-sensitive and the actual field is 'Department' with capital D

B The collection is empty

C The query syntax is invalid and causes an error

D The database connection is lost

💡 Hint

Check exact spelling and case of field names in documents.

MongoDB field names are case-sensitive. If the field is 'Department', querying 'department' returns no matches.

🧠 Conceptual

expert

Understanding find method cursor behavior

What does the find() method return in MongoDB when called without toArray() or iteration?

A A cursor object that points to the matching documents but does not fetch them immediately

B An array of all matching documents immediately

C A single document matching the query

D An error because find() must be followed by toArray()

💡 Hint

Think about how MongoDB handles large result sets efficiently.

The find() method returns a cursor, which is a pointer to the result set. Documents are fetched as needed.

Practice

1. What does the find method do in MongoDB?

easy

A. It deletes documents from a collection.

B. It updates documents in a collection.

C. It searches for documents that match a query.

D. It creates a new collection.

Solution

1. Step 1: Understand the purpose of find

   The find method is used to search for documents in a collection that match a given query.

2. Step 2: Compare with other operations

   Deleting, updating, or creating collections are done by other methods like deleteOne, updateOne, or createCollection.

3. Final Answer:

   It searches for documents that match a query. -> Option C

4. Quick Check:

   find = search documents [OK]

Hint: Remember: find means search for matching documents [OK]

Common Mistakes:

• Confusing find with delete or update methods
• Thinking find creates collections
• Assuming find modifies documents

2. Which of the following is the correct syntax to find all documents in a collection named users?

easy

A. db.users.find({})

B. db.users.findAll()

C. db.users.search({})

D. db.users.get()

Solution

1. Step 1: Recall the correct method name and syntax

   The correct method to find documents is find, and to find all documents, we pass an empty query {}.

2. Step 2: Check other options for validity

   findAll, search, and get are not valid MongoDB methods.

3. Final Answer:

   db.users.find({}) -> Option A

4. Quick Check:

   Use find({}) to get all documents [OK]

Hint: Use empty braces {} inside find() to get all documents [OK]

Common Mistakes:

• Using non-existent methods like findAll or search
• Omitting parentheses after find
• Passing wrong arguments to find

3. Given the collection products with documents:
{ name: "Pen", price: 5 }
{ name: "Book", price: 15 }
What will db.products.find({ price: { $lt: 10 } }).toArray() return?

medium

A. [{ name: "Pen", price: 5 }]

B. [{ name: "Book", price: 15 }]

C. []

D. [{ name: "Pen", price: 5 }, { name: "Book", price: 15 }]

Solution

1. Step 1: Understand the query filter

   The query { price: { $lt: 10 } } means find documents where price is less than 10.

2. Step 2: Check documents against the filter

   "Pen" has price 5 which is less than 10, "Book" has price 15 which is not less than 10.

3. Final Answer:

   [{ name: "Pen", price: 5 }] -> Option A

4. Quick Check:

   Price < 10 returns only Pen [OK]

Hint: Use $lt to filter values less than a number [OK]

Common Mistakes:

• Confusing $lt with $gt
• Expecting all documents to return
• Not converting cursor to array before viewing

4. What is wrong with this query?
db.orders.find(price: 100)

medium

A. The query should use double quotes around price.

B. The collection name is incorrect.

C. The find method cannot filter by price.

D. Missing curly braces around the query object.

Solution

1. Step 1: Check the syntax of the find method

   The query argument to find must be an object enclosed in curly braces {}.

2. Step 2: Identify the missing braces

   The query is written as price: 100 without braces, which is invalid syntax.

3. Final Answer:

   Missing curly braces around the query object. -> Option D

4. Quick Check:

   Query must be inside {} in find() [OK]

Hint: Always wrap query in curly braces {} inside find() [OK]

Common Mistakes:

• Omitting curly braces around query
• Using wrong collection name
• Thinking quotes are mandatory around keys

5. You want to find all documents in employees collection but only show their name and department fields. Which query is correct?

hard

A. db.employees.find({ name: 1, department: 1 })

B. db.employees.find({}, { name: 1, department: 1, _id: 0 })

C. db.employees.find({}, { name: 1, department: 1 })

D. db.employees.find({ name: 1, department: 1, _id: 0 })

Solution

1. Step 1: Understand projection in find()

   Projection is the second argument to find and specifies which fields to include (1) or exclude (0).

2. Step 2: Check the correct syntax for showing only name and department

   Use { name: 1, department: 1, _id: 0 } to include those fields and exclude the _id field.

3. Final Answer:

   db.employees.find({}, { name: 1, department: 1, _id: 0 }) -> Option B

4. Quick Check:

   Projection is second arg with 1 to include fields [OK]

Hint: Use second argument in find() to project fields [OK]

Common Mistakes:

• Putting projection inside the query object
• Not excluding _id when projecting
• Using query to filter fields instead of projection