Bulk write operations in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this bulkWrite operation?

Consider a MongoDB collection products with documents:

[{ _id: 1, name: "Pen", stock: 10 }, { _id: 2, name: "Pencil", stock: 5 }]

What will be the result of this bulkWrite operation?

db.products.bulkWrite([
  { updateOne: { filter: { _id: 1 }, update: { $inc: { stock: 5 } } } },
  { updateOne: { filter: { _id: 2 }, update: { $set: { stock: 0 } } } }
])

MongoDB

db.products.bulkWrite([
  { updateOne: { filter: { _id: 1 }, update: { $inc: { stock: 5 } } } },
  { updateOne: { filter: { _id: 2 }, update: { $set: { stock: 0 } } } }
])

A {"nMatched":2,"nModified":2,"ok":1}

B {"nMatched":1,"nModified":1,"ok":1}

C {"nMatched":2,"nModified":1,"ok":1}

D {"nMatched":1,"nModified":2,"ok":1}

💡 Hint

Each updateOne matches one document and modifies it.

Both updateOne operations match one document each (_id 1 and 2). Both modify the stock field, so nMatched and nModified are 2.

📝 Syntax

intermediate

Which bulkWrite syntax is valid to insert two new documents?

Choose the correct bulkWrite syntax to insert two new documents into a MongoDB collection.

A db.collection.bulkWrite([{ insertOne: { doc: { name: "A" } } }, { insertOne: { doc: { name: "B" } } }])

B db.collection.bulkWrite([{ insertOne: { document: { name: "A" } } }, { insertOne: { document: { name: "B" } } }])

C db.collection.bulkWrite([{ insertOne: { document: { name: "A" } } }, { insertMany: { documents: [{ name: "B" }] } }])

D db.collection.bulkWrite([{ insert: { document: { name: "A" } } }, { insert: { document: { name: "B" } } }])

💡 Hint

The correct key for inserting a single document is insertOne with document field.

Option B uses the correct insertOne operation with document key. Options A and B use invalid keys. Option B mixes insertOne and insertMany which is valid in bulkWrite.

❓ optimization

advanced

How to optimize bulkWrite for mixed inserts and updates?

You want to insert 100 new documents and update 50 existing documents in one bulkWrite operation. Which approach is best for performance?

A Use one bulkWrite with 150 operations mixing insertOne and updateOne commands.

B Run 100 insertMany operations separately, then 50 updateOne operations separately.

C Use separate bulkWrite calls: one for all inserts, one for all updates.

D Use a single updateMany operation for all updates and insertOne for each insert.

💡 Hint

bulkWrite supports mixing different operation types in one call.

Option A uses one bulkWrite call with all operations mixed, reducing network overhead and improving performance. Options B and C cause multiple calls. Option A is invalid because updateMany cannot update different documents with different filters in one call.

🔧 Debug

advanced

Why does this bulkWrite operation fail with a syntax error?

Identify the syntax error in this bulkWrite command:

db.collection.bulkWrite([
  { updateOne: { filter: { _id: 1 }, update: { $set: { name: "X" } } } },
  { insertOne: { document: { name: "Y" } }
])

MongoDB

db.collection.bulkWrite([
  { updateOne: { filter: { _id: 1 }, update: { $set: { name: "X" } } } },
  { insertOne: { document: { name: "Y" } }
])

A Invalid use of $set operator in updateOne.

B Missing comma between updateOne and insertOne operations.

C Using bulkWrite without a callback causes syntax error.

D Missing closing brace '}' for the insertOne operation object.

💡 Hint

Check the braces and commas carefully in the array of operations.

The insertOne operation object is missing a closing brace '}', causing a syntax error. The comma between operations is present. $set is valid. bulkWrite can be used without callback in modern drivers.

🧠 Conceptual

expert

What happens if an unordered bulkWrite operation fails midway?

You run a bulkWrite with ordered: false containing 10 operations. The 5th operation fails due to a duplicate key error. What is the behavior of the remaining operations?

A Only operations before the failure run; operations after are skipped.

B Execution stops immediately after the failure; remaining operations are not run.

C All other operations continue to execute despite the failure.

D The entire bulkWrite is rolled back; no changes are applied.

💡 Hint

Consider the difference between ordered and unordered bulkWrite behavior.

With ordered: false, bulkWrite continues executing all operations even if some fail. Only the failed operations are reported as errors. With ordered: true, execution stops at the first failure.

Practice

1. What is the main advantage of using bulkWrite() in MongoDB?

easy

A. It allows multiple write operations in a single request, improving performance.

B. It automatically backs up the database before each write.

C. It only works with read operations.

D. It requires separate calls for each write operation.

Solution

1. Step 1: Understand what bulkWrite() does

   bulkWrite() lets you send many write commands (insert, update, delete) in one call.

2. Step 2: Identify the benefit

   Sending many operations at once reduces network overhead and speeds up database changes.

3. Final Answer:

   It allows multiple write operations in a single request, improving performance. -> Option A

4. Quick Check:

   Bulk write = multiple writes in one call [OK]

Hint: Bulk write = many writes at once for speed [OK]

Common Mistakes:

• Thinking bulkWrite only reads data
• Believing it backs up data automatically
• Assuming it requires separate calls per operation

2. Which of the following is the correct syntax to perform a bulk write with an insert and an update in MongoDB?

easy

A. db.collection.bulkWrite([{ insertOne: { document: { name: 'Alice' } } }, { updateOne: { filter: { name: 'Bob' }, update: { $set: { age: 30 } } } }])

B. db.collection.bulkWrite(insertOne: { document: { name: 'Alice' } }, updateOne: { filter: { name: 'Bob' }, update: { $set: { age: 30 } } })

C. db.collection.bulkWrite([{ insert: { name: 'Alice' } }, { update: { name: 'Bob', age: 30 } }])

D. db.collection.bulkWrite([{ insertOne: { name: 'Alice' } }, { updateOne: { name: 'Bob', age: 30 } }])

Solution

1. Step 1: Check the bulkWrite syntax

   bulkWrite expects an array of operation objects like { insertOne: { document: {...} } } or { updateOne: { filter: {...}, update: {...} } }.

2. Step 2: Match options to syntax

   db.collection.bulkWrite([{ insertOne: { document: { name: 'Alice' } } }, { updateOne: { filter: { name: 'Bob' }, update: { $set: { age: 30 } } } }]) correctly uses insertOne and updateOne with proper nested objects. Others miss brackets or use wrong keys.

3. Final Answer:

   db.collection.bulkWrite([{ insertOne: { document: { name: 'Alice' } } }, { updateOne: { filter: { name: 'Bob' }, update: { $set: { age: 30 } } } }]) -> Option A

4. Quick Check:

   Correct bulkWrite syntax = db.collection.bulkWrite([{ insertOne: { document: { name: 'Alice' } } }, { updateOne: { filter: { name: 'Bob' }, update: { $set: { age: 30 } } } }]) [OK]

Hint: bulkWrite needs array of operation objects [OK]

Common Mistakes:

• Omitting the array brackets []
• Using wrong keys like 'insert' instead of 'insertOne'
• Not nesting filter and update properly

3. Given the following bulkWrite operation, what will be the result in the collection?

db.users.bulkWrite([
  { insertOne: { document: { _id: 1, name: 'John' } } },
  { updateOne: { filter: { _id: 1 }, update: { $set: { age: 25 } } } },
  { deleteOne: { filter: { name: 'John' } } }
])

medium

A. The collection will have one document: { _id: 1, name: 'John', age: 25 }

B. The operation will fail due to conflicting commands.

C. The collection will be empty after the operations.

D. The collection will have one document: { _id: 1, name: 'John' } without age.

Solution

1. Step 1: Analyze each bulkWrite operation in order

   First, insert { _id: 1, name: 'John' }. Second, update that document to add age: 25. Third, delete the document where name is 'John'.

2. Step 2: Determine final collection state

   After deletion, the document is removed, so the collection is empty.

3. Final Answer:

   The collection will be empty after the operations. -> Option C

4. Quick Check:

   Insert + update + delete = empty collection [OK]

Hint: Last delete removes the document, leaving collection empty [OK]

Common Mistakes:

• Assuming update adds a new document
• Ignoring the delete operation
• Thinking operations run independently

4. Identify the error in this bulkWrite operation:

db.products.bulkWrite([
  { insertOne: { document: { _id: 101, name: 'Pen', price: 10 } } },
  { updateOne: { filter: { _id: 101 }, update: { name: 'Pencil' } } }
])

medium

A. The filter in updateOne is incorrect.

B. The insertOne operation is missing the document key.

C. bulkWrite cannot mix insertOne and updateOne operations.

D. The update document is missing the $set operator.

Solution

1. Step 1: Review the updateOne operation

   The update document must use an update operator like $set to modify fields.

2. Step 2: Identify the missing operator

   Here, { name: 'Pencil' } is used directly, which replaces the entire document (losing the price field). It should be { $set: { name: 'Pencil' } }.

3. Final Answer:

   The update document is missing the $set operator. -> Option D

4. Quick Check:

   Update needs $set for field changes [OK]

Hint: Always use $set in update documents [OK]

Common Mistakes:

• Forgetting $set in updateOne
• Thinking insertOne needs $set
• Believing bulkWrite can't mix operations

5. You want to update multiple documents in a collection using bulkWrite(). You have these operations:

• Update user with _id 1 to set age to 30
• Update user with _id 2 to set age to 25
• Insert a new user with _id 3 and name 'Anna'

Which bulkWrite array correctly performs all these changes in one call?

hard

A. [
{ updateMany: { filter: { _id: { $in: [1, 2] } }, update: { $set: { age: 30 } } } },
{ insertOne: { document: { _id: 3, name: 'Anna' } } }
]

B. [
{ updateOne: { filter: { _id: 1 }, update: { $set: { age: 30 } } } },
{ updateOne: { filter: { _id: 2 }, update: { $set: { age: 25 } } } },
{ insertOne: { document: { _id: 3, name: 'Anna' } } }
]

C. [
{ updateOne: { filter: { _id: 1 }, update: { age: 30 } } },
{ updateOne: { filter: { _id: 2 }, update: { age: 25 } } },
{ insertOne: { document: { _id: 3, name: 'Anna' } } }
]

D. [
{ updateOne: { filter: { _id: 1 }, update: { $set: { age: 30 } } } },
{ updateOne: { filter: { _id: 2 }, update: { $set: { age: 25 } } } },
{ insertOne: { document: { name: 'Anna' } } }
]

Solution

1. Step 1: Check each updateOne operation

   Both updates correctly use updateOne with $set to change age for _id 1 and 2.

2. Step 2: Verify insertOne operation

   [ { updateOne: { filter: { _id: 1 }, update: { $set: { age: 30 } } } }, { updateOne: { filter: { _id: 2 }, update: { $set: { age: 25 } } } }, { insertOne: { document: { _id: 3, name: 'Anna' } } } ] inserts a new document with _id 3 and name 'Anna', which matches the requirement.

3. Step 3: Compare with other options

   [ { updateMany: { filter: { _id: { $in: [1, 2] } }, update: { $set: { age: 30 } } } }, { insertOne: { document: { _id: 3, name: 'Anna' } } } ] uses updateMany incorrectly (age 30 for both, but second should be 25). [ { updateOne: { filter: { _id: 1 }, update: { age: 30 } } }, { updateOne: { filter: { _id: 2 }, update: { age: 25 } } }, { insertOne: { document: { _id: 3, name: 'Anna' } } } ] misses $set. [ { updateOne: { filter: { _id: 1 }, update: { $set: { age: 30 } } } }, { updateOne: { filter: { _id: 2 }, update: { $set: { age: 25 } } } }, { insertOne: { document: { name: 'Anna' } } } ] misses _id in insert.

4. Final Answer:

   [ { updateOne: { filter: { _id: 1 }, update: { $set: { age: 30 } } } }, { updateOne: { filter: { _id: 2 }, update: { $set: { age: 25 } } } }, { insertOne: { document: { _id: 3, name: 'Anna' } } } ] -> Option B

5. Quick Check:

   Use updateOne with $set and insertOne with full document [OK]

Hint: Use updateOne with $set and insertOne with full document [OK]

Common Mistakes:

• Using update without $set
• Missing _id in insert document
• Using updateMany when individual updates differ