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.

Practice

1. What is the main purpose of an election in a MongoDB replica set?

easy

A. To select a primary server that manages writes

B. To delete old data from the database

C. To backup the database automatically

D. To create new collections in the database

Solution

1. Step 1: Understand the role of election in MongoDB

   In a replica set, election chooses a primary server to handle write operations.

2. Step 2: Identify the correct purpose

   The primary server manages writes and coordinates data replication.

3. Final Answer:

   To select a primary server that manages writes -> Option A

4. Quick Check:

   Election = select primary server [OK]

Hint: Election picks the primary server for writes [OK]

Common Mistakes:

• Thinking election deletes data
• Confusing election with backup process
• Believing election creates collections

2. Which MongoDB command shows the current election status in a replica set?

easy

A. db.showElection()

B. db.electionStatus()

C. rs.status()

D. rs.election()

Solution

1. Step 1: Recall the command to check replica set status

   The command rs.status() shows detailed replica set info including election status.

2. Step 2: Verify other options

   Other commands like db.showElection() or rs.election() do not exist in MongoDB.

3. Final Answer:

   rs.status() -> Option C

4. Quick Check:

   Check election with rs.status() [OK]

Hint: Use rs.status() to see election info [OK]

Common Mistakes:

• Using non-existent commands
• Confusing db commands with rs commands
• Trying to use election-specific commands that don't exist

3. Given the following output from rs.status(), which member is currently the primary?

{
  "members": [
    {"_id": 0, "stateStr": "SECONDARY"},
    {"_id": 1, "stateStr": "PRIMARY"},
    {"_id": 2, "stateStr": "SECONDARY"}
  ]
}

medium

A. Member with _id 0

B. No primary elected yet

C. Member with _id 2

D. Member with _id 1

Solution

1. Step 1: Read the stateStr values for each member

   Member with _id 1 has stateStr "PRIMARY", others are "SECONDARY".

2. Step 2: Identify the primary member

   The member with stateStr "PRIMARY" is the current primary server.

3. Final Answer:

   Member with _id 1 -> Option D

4. Quick Check:

   Primary = stateStr "PRIMARY" member [OK]

Hint: Primary member has stateStr 'PRIMARY' in rs.status() [OK]

Common Mistakes:

• Choosing a secondary member
• Assuming no primary if unsure
• Ignoring the stateStr field

4. You run rs.status() but get an error: not authorized on admin to execute command. What is the likely cause?

medium

A. You lack proper user permissions to run rs.status()

B. You are not connected to the primary server

C. The replica set is not initialized

D. The command rs.status() is misspelled

Solution

1. Step 1: Understand the error message

   The error says 'not authorized', meaning permission is denied.

2. Step 2: Match error to cause

   Lack of user permissions to run the command causes this error, not connection or spelling issues.

3. Final Answer:

   You lack proper user permissions to run rs.status() -> Option A

4. Quick Check:

   Authorization error = permission issue [OK]

Hint: Authorization errors mean missing user permissions [OK]

Common Mistakes:

• Assuming connection problem
• Thinking replica set not initialized
• Blaming command spelling

5. In a MongoDB replica set with 5 members, what minimum number of votes is needed to elect a primary during an election?

hard

A. 2 votes

B. 3 votes

C. 4 votes

D. 5 votes

Solution

1. Step 1: Recall election voting rules in MongoDB

   Primary election requires a majority of votes from members.

2. Step 2: Calculate majority for 5 members

   Majority is more than half, so 3 out of 5 votes are needed.

3. Final Answer:

   3 votes -> Option B

4. Quick Check:

   Majority of 5 = 3 votes [OK]

Hint: Majority votes needed: half members + 1 [OK]

Common Mistakes:

• Choosing half votes instead of majority
• Thinking all members must vote
• Confusing votes with number of members