Election process concept in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find candidates with more than 1000 votes

Given a collection candidates with documents containing name and votes, which query returns all candidates with votes greater than 1000?

MongoDB

db.candidates.find({ votes: { $gt: 1000 } })

A db.candidates.find({ votes: { $gt: 1000 } })

B db.candidates.find({ votes: { $lt: 1000 } })

C db.candidates.find({ votes: { $gte: 1000 } })

D db.candidates.find({ votes: 1000 })

💡 Hint

Use the $gt operator to find values greater than a number.

The $gt operator selects documents where the value of the field is greater than the specified value. Here, it returns candidates with votes more than 1000.

❓ query_result

intermediate

Count total votes for all candidates

Which aggregation pipeline correctly sums the votes field for all candidates in the candidates collection?

MongoDB

db.candidates.aggregate([{ $group: { _id: null, totalVotes: { $sum: "$votes" } } }])

A db.candidates.aggregate([{ $group: { _id: "$votes", totalVotes: { $count: {} } } }])

B db.candidates.aggregate([{ $sum: "$votes" }])

C db.candidates.aggregate([{ $group: { _id: null, totalVotes: { $avg: "$votes" } } }])

D db.candidates.aggregate([{ $group: { _id: null, totalVotes: { $sum: "$votes" } } }])

💡 Hint

Use $group stage with $sum accumulator to add values.

The $group stage groups all documents (using _id:null) and sums the votes field to get total votes.

📝 Syntax

advanced

Identify the syntax error in this update query

What error does this MongoDB update query produce?

db.candidates.updateOne({ name: "Alice" }, { $set: { votes: votes + 1 } })

A SyntaxError: Cannot use field name 'votes' directly in update document

B TypeError: votes is undefined

C No error, query runs successfully

D RuntimeError: Invalid update operator

💡 Hint

In update documents, you cannot use field names as variables directly.

The update document tries to increment votes by using 'votes + 1' directly, which is invalid syntax. MongoDB requires $inc operator for increments.

❓ query_result

advanced

Find the candidate with the highest votes

Which query returns the candidate document with the highest number of votes?

MongoDB

db.candidates.find().sort({ votes: -1 }).limit(1)

A db.candidates.aggregate([{ $sort: { votes: -1 } }, { $limit: 1 }])

B db.candidates.aggregate([{ $sort: { votes: 1 } }, { $limit: 1 }])

C db.candidates.find().sort({ votes: -1 }).limit(1)

D db.candidates.find().sort({ votes: 1 }).limit(1)

💡 Hint

Sort descending by votes and limit to 1 document.

Sorting by votes descending (-1) and limiting to 1 returns the candidate with the highest votes.

🧠 Conceptual

expert

Election process: Ensuring vote uniqueness

In a MongoDB collection votes where each document records a voterId and candidateId, which approach best ensures that each voter can only vote once?

A Use a compound index on <code>voterId</code> and <code>candidateId</code> without uniqueness

B Create a unique index on the <code>voterId</code> field

C Manually check for existing votes before inserting a new vote document

D Allow multiple votes and filter duplicates in application code

💡 Hint

Indexes can enforce uniqueness at the database level.

A unique index on voterId prevents inserting multiple documents with the same voterId, ensuring one vote per voter.