Single document atomicity in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to update a single field atomically in a MongoDB document.

MongoDB

db.collection.updateOne({ _id: 1 }, { [1]: { score: 100 } })

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

A$push

B$inc

C$set

D$unset

Attempts:

3 left

💡 Hint

Common Mistakes

Using $inc when you want to set a specific value.

Using $push which is for arrays, not single fields.

The $set operator updates the value of a field atomically in a single document.

2fill in blank

medium

Complete the code to increment a numeric field atomically in a MongoDB document.

MongoDB

db.collection.updateOne({ _id: 2 }, { [1]: { visits: 1 } })

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

A$set

B$addToSet

C$push

D$inc

Attempts:

3 left

💡 Hint

Common Mistakes

Using $set which replaces the value instead of incrementing.

Using $push which is for arrays.

The $inc operator increments a numeric field atomically by the specified value.

3fill in blank

hard

Fix the error in the update command to atomically add a value to an array field without duplicates.

MongoDB

db.collection.updateOne({ _id: 3 }, { [1]: { tags: "mongodb" } })

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

A$push

B$addToSet

C$set

D$inc

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push which can add duplicates.

Using $set which replaces the whole array.

The $addToSet operator adds a value to an array only if it does not already exist, ensuring atomicity.

4fill in blank

hard

Fill both blanks to atomically update multiple fields in a single document.

MongoDB

db.collection.updateOne({ _id: 4 }, { [1]: { score: 90, level: 5 } })

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

A$set

B$inc

C$unset

D$push

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing $set with $inc or other operators incorrectly.

Using $unset which removes fields instead of updating.

Using $set for both fields updates them atomically in a single document.

5fill in blank

hard

Fill all three blanks to atomically update a document: increment visits, add a tag uniquely, and set status.

MongoDB

db.collection.updateOne({ _id: 5 }, { [1]: { visits: 1 }, [2]: { tags: "new" }, [3]: { status: "active" } })

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

A$inc

B$addToSet

C$set

D$push

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push instead of $addToSet causing duplicates.

Using $set to increment numbers which replaces values.

Use $inc to increment visits, $addToSet to add unique tags, and $set to update status atomically.

Practice

1. What does single document atomicity mean in MongoDB?

easy

A. All changes to a single document happen completely or not at all

B. Multiple documents are updated at the same time atomically

C. Only one field in a document can be updated at a time

D. Documents are locked during any update operation

Solution

1. Step 1: Understand atomicity in MongoDB context

   MongoDB guarantees that updates to a single document are atomic, meaning all changes in that update happen fully or not at all.

2. Step 2: Compare options with this definition

   All changes to a single document happen completely or not at all correctly states this. Options A, B, and D describe behaviors that are not true for single document atomicity.

3. Final Answer:

   All changes to a single document happen completely or not at all -> Option A

4. Quick Check:

   Single document atomicity = All-or-nothing update [OK]

Hint: Atomic means all or nothing for one document update [OK]

Common Mistakes:

• Thinking atomicity applies to multiple documents
• Believing only one field updates atomically
• Assuming document locking happens automatically

2. Which of the following is the correct syntax to update a single document atomically in MongoDB?

easy

A. db.collection.update({ _id: 1 }, { name: 'Alice' })

B. db.collection.updateMany({ _id: 1 }, { $set: { name: 'Alice' } })

C. db.collection.modifyOne({ _id: 1 }, { $set: { name: 'Alice' } })

D. db.collection.updateOne({ _id: 1 }, { $set: { name: 'Alice' } })

Solution

1. Step 1: Identify the correct update method for one document

   The method updateOne updates a single document atomically. It uses a filter and an update operator like $set.

2. Step 2: Check each option's syntax

   db.collection.updateOne({ _id: 1 }, { $set: { name: 'Alice' } }) uses updateOne with $set, which is correct. db.collection.update({ _id: 1 }, { name: 'Alice' }) uses deprecated update without update operator. db.collection.modifyOne({ _id: 1 }, { $set: { name: 'Alice' } }) uses a non-existent method modifyOne. db.collection.updateMany({ _id: 1 }, { $set: { name: 'Alice' } }) updates many documents, not one.

3. Final Answer:

   db.collection.updateOne({ _id: 1 }, { $set: { name: 'Alice' } }) -> Option D

4. Quick Check:

   Use updateOne with $set for atomic single document update [OK]

Hint: Use updateOne with $set to update one document atomically [OK]

Common Mistakes:

• Using update without $set operator
• Using updateMany when only one document should update
• Using non-existent methods like modifyOne

3. Given the document { _id: 1, count: 5 }, what will be the document after this atomic update?

db.collection.updateOne({ _id: 1 }, { $inc: { count: 3 }, $set: { status: 'active' } })

medium

A. { _id: 1, count: 3, status: 'active' }

B. { _id: 1, count: 8, status: 'active' }

C. { _id: 1, count: 5 }

D. Update fails with error

Solution

1. Step 1: Understand the update operators used

   The $inc operator increases the count field by 3. The $set operator adds or updates the status field to 'active'.

2. Step 2: Calculate the new document values

   Original count is 5, incremented by 3 gives 8. The status field is set to 'active'.

3. Final Answer:

   { _id: 1, count: 8, status: 'active' } -> Option B

4. Quick Check:

   $inc adds 3, $set adds status [OK]

Hint: Combine $inc and $set to update fields atomically [OK]

Common Mistakes:

• Thinking $inc replaces the field instead of adding
• Ignoring $set operator effect
• Assuming update fails without errors

4. You wrote this update but it does not work as expected:

db.collection.updateOne({ _id: 2 }, { count: 10, status: 'done' })

What is the problem?

medium

A. The collection name is invalid

B. The filter {_id: 2} is incorrect syntax

C. Missing update operator like $set causes the update to replace the entire document

D. updateOne cannot update multiple fields at once

Solution

1. Step 1: Check the update document structure

   The update document must use update operators like $set to specify fields to change. Here, the update document lacks any operator.

2. Step 2: Understand MongoDB update syntax rules

   Without an operator, MongoDB treats the update document as a replacement document, overwriting the entire document. This causes unexpected behavior if other fields exist.

3. Final Answer:

   Missing update operator like $set causes the update to replace the entire document -> Option C

4. Quick Check:

   Always use $set or other operators in update document [OK]

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

Common Mistakes:

• Forgetting $set causes full document replacement
• Assuming updateOne updates fields without operators
• Misunderstanding filter syntax

5. You want to update a user's profile atomically to change their email and increment their login count. Which update operation ensures atomicity and correctness?

db.users.updateOne(
  { _id: 123 },
  { $set: { email: '[e-mail]' }, $inc: { logins: 1 } }
)

Why is this update atomic and what would happen if you tried to update email and logins in two separate updateOne calls?

hard

A. This update is atomic because both changes happen together; separate calls risk partial updates

B. This update is not atomic; MongoDB cannot update two fields at once

C. Separate updateOne calls are better for performance and atomicity

D. Atomicity only applies to single field updates, so this is invalid

Solution

1. Step 1: Understand atomicity of single document updates

   MongoDB guarantees that all changes in a single updateOne call on one document happen fully or not at all, ensuring atomicity.

2. Step 2: Compare single update vs multiple updates

   Using one updateOne with both $set and $inc ensures both fields change together. Using two separate updateOne calls risks one succeeding and the other failing, causing inconsistent data.

3. Final Answer:

   This update is atomic because both changes happen together; separate calls risk partial updates -> Option A

4. Quick Check:

   Single updateOne call = atomic multi-field update [OK]

Hint: Combine changes in one updateOne call for atomicity [OK]

Common Mistakes:

• Thinking atomicity applies across multiple update calls
• Believing MongoDB can't update multiple fields atomically
• Assuming separate updates are safer