Array update with positional $ operator in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Update a specific array element using the positional $ operator

Given the collection students with documents like:

{ "_id": 1, "name": "Alice", "grades": [85, 90, 78] }

Which update query will change the first grade equal to 90 to 95?

MongoDB

db.students.updateOne({ _id: 1, grades: 90 }, { $set: { "grades.$": 95 } })

A db.students.updateOne({ _id: 1 }, { $set: { "grades.$": 95 } })

B db.students.updateOne({ _id: 1, grades: 90 }, { $set: { "grades.$": 95 } })

C db.students.updateOne({ _id: 1, grades: { $elemMatch: { $eq: 90 } } }, { $set: { "grades.$": 95 } })

D db.students.updateOne({ _id: 1, grades: { $eq: 90 } }, { $set: { "grades.$": 95 } })

💡 Hint

The positional $ operator requires the query to match the array element directly.

Option B correctly matches the document with _id 1 and an array element grades equal to 90, then uses the positional $ operator to update that element to 95. Option B lacks the array element match, so $ cannot identify which element to update. Option B uses $elemMatch incorrectly with a direct value. Option B will not match the document because grades: {$eq: 90} requires the entire grades field to equal 90.

❓ query_result

intermediate

Result of updating multiple array elements with positional $ operator

Consider the document:

{ "_id": 2, "tasks": [ { "name": "task1", "done": false }, { "name": "task2", "done": false } ] }

What will be the value of tasks after running:

db.collection.updateOne({ _id: 2, "tasks.done": false }, { $set: { "tasks.$.done": true } })

?

A No change, update fails

B [ { "name": "task1", "done": true }, { "name": "task2", "done": true } ]

C [ { "name": "task1", "done": false }, { "name": "task2", "done": true } ]

D [ { "name": "task1", "done": true }, { "name": "task2", "done": false } ]

💡 Hint

The positional $ operator updates only the first matching array element.

The query matches the first element in tasks where done is false (task1). The update sets that element's done to true. The second element remains unchanged. So only task1.done becomes true.

📝 Syntax

advanced

Identify the syntax error in using positional $ operator

Which update query will cause a syntax error when trying to update an array element using the positional $ operator?

A db.collection.updateOne({ "items.$": 10 }, { $set: { "items.$.value": 20 } })

B )} } 02 :"eulav.$.smeti" { :tes$ { ,} 01 :"eulav.smeti" {(enOetadpu.noitcelloc.bd

C b.collection.updateOne({ "items.value": 10 }, { $set: { "items.$.value": 20 } })

D db.collection.updateOne({ "items.value": 10 }, { $set: { "items.$.value": 20 } })

💡 Hint

The positional $ operator cannot be used inside the query filter like this.

Option A uses "items.$" in the query filter, which is invalid syntax. The positional $ operator is only valid in the update document, not in the query filter. Options A, B, and D are syntactically valid.

❓ optimization

advanced

Optimizing array element update with positional $ operator

You want to update the status field to 'completed' for the first task with done: false in documents where userId is 123. Which query is the most efficient?

A db.tasks.updateOne({ userId: 123 }, { $set: { "tasks.$.status": "completed" } })

B db.tasks.updateMany({ userId: 123 }, { $set: { "tasks.$[elem].status": "completed" } }, { arrayFilters: [ { "elem.done": false } ] })

C db.tasks.updateOne({ userId: 123, "tasks.done": false }, { $set: { "tasks.$.status": "completed" } })

D db.tasks.updateMany({ userId: 123, "tasks.done": false }, { $set: { "tasks.$.status": "completed" } })

💡 Hint

Use the positional $ operator with a filter that matches the array element.

Option C updates only one document matching userId 123 and the first task with done false, setting its status to completed. Option C updates multiple documents and uses arrayFilters which is more complex and affects multiple elements. Option C lacks the array element match in the query, so $ cannot identify which element to update. Option C updates multiple documents but $ operator only updates the first matching element per document, which may be inefficient if only one update is needed.

🧠 Conceptual

expert

Understanding limitations of the positional $ operator

Which statement about the positional $ operator in MongoDB updates is TRUE?

A The positional $ operator requires the query filter to match the array element to update.

B The positional $ operator can update multiple matching elements in an array at once.

C The positional $ operator can be used in aggregation pipeline stages to update arrays.

D The positional $ operator can be used without specifying the array field in the query filter.

💡 Hint

Think about how MongoDB identifies which array element to update.

Option A is true: the positional $ operator updates the first array element matched by the query filter. Option A is false because $ updates only one element per matched document. Option A is false because $ is used in update documents, not aggregation pipelines. Option A is false because the query filter must include the array field to identify the element to update.

Practice

1.

What does the positional $ operator do in a MongoDB update?

easy

A. It updates the first matching element in an array field.

B. It deletes the entire array field.

C. It adds a new element to the array.

D. It replaces the whole document.

Solution

1. Step 1: Understand the positional operator usage

   The $ operator targets the first array element that matches the query condition.

2. Step 2: Recognize update effect

   It updates only that matched element without changing other array elements or the whole array.

3. Final Answer:

   It updates the first matching element in an array field. -> Option A

4. Quick Check:

   Positional $ updates one array item [OK]

Hint: Positional $ updates only the first matched array element [OK]

Common Mistakes:

• Thinking $ replaces the whole array
• Confusing $ with $push or $pop
• Assuming it updates all matching elements

2.

Which of the following is the correct syntax to update the first matching element in the scores array where score is 85?

db.students.updateOne(
  { "scores.score": 85 },
  { $set: { "scores.$.score": 90 } }
)

easy

A. db.students.updateOne({ "scores.score": 85 }, { $set: { "scores.score": 90 } })

B. db.students.updateOne({ "scores.$.score": 85 }, { $set: { "scores.$.score": 90 } })

C. db.students.updateOne({ "scores.score": 85 }, { $set: { "scores.$.score": 90 } })

D. db.students.updateOne({ "scores.$.score": 85 }, { $set: { "scores.score": 90 } })

Solution

1. Step 1: Identify the query filter

   The filter must be on the array field without the positional operator: { "scores.score": 85 }.

2. Step 2: Use positional operator in update

   The update uses "scores.$.score" to update the matched element's score.

3. Final Answer:

   db.students.updateOne({ "scores.score": 85 }, { $set: { "scores.$.score": 90 } }) -> Option C

4. Quick Check:

   Query without $; update with scores.$ [OK]

Hint: Use $ only in update path, not in query filter [OK]

Common Mistakes:

• Using $ in the query filter
• Not using $ in the update path
• Updating the whole array instead of one element

3.

Given the document:

{ _id: 1, items: [ { name: "apple", qty: 5 }, { name: "banana", qty: 3 } ] }

What will be the document after running this update?

db.collection.updateOne(
  { _id: 1, "items.name": "banana" },
  { $set: { "items.$.qty": 10 } }
)

medium

A. { _id: 1, items: [ { name: "apple", qty: 5 }, { name: "banana", qty: 10 } ] }

B. { _id: 1, items: [ { name: "apple", qty: 10 }, { name: "banana", qty: 3 } ] }

C. { _id: 1, items: [ { name: "apple", qty: 5 }, { name: "banana", qty: 3 }, { name: "banana", qty: 10 } ] }

D. { _id: 1, items: [ { name: "apple", qty: 5 } ] }

Solution

1. Step 1: Find matching array element

   The query matches document with _id 1 and array element where name is "banana".

2. Step 2: Update matched element's qty

   The positional operator $ updates the qty of the matched "banana" element to 10.

3. Final Answer:

   { _id: 1, items: [ { name: "apple", qty: 5 }, { name: "banana", qty: 10 } ] } -> Option A

4. Quick Check:

   Only matched array element qty changed [OK]

Hint: Positional $ updates only matched array element [OK]

Common Mistakes:

• Updating wrong array element
• Adding new element instead of updating
• Replacing entire array

4.

What is wrong with this update query?

db.orders.updateOne(
  { "products.$.id": 123 },
  { $set: { "products.$.quantity": 5 } }
)

medium

A. Using positional operator $ in the query filter is invalid.

B. Both A and C are correct.

C. The query filter should use products.id without $.

D. The $set operator cannot update array elements.

Solution

1. Step 1: Check query filter syntax

   The positional operator $ cannot be used in the query filter path; it must be omitted.

2. Step 2: Correct query filter

   The filter should be { "products.id": 123 } to find the matching array element.

3. Final Answer:

   Both A and C are correct. -> Option B

4. Quick Check:

   No $ in query filter; only in update path [OK]

Hint: Never use $ in query filter, only in update path [OK]

Common Mistakes:

• Using $ in query filter
• Confusing query filter and update syntax
• Assuming $set can't update arrays

5.

You have documents like:

{ _id: 1, tags: [ { name: "urgent", status: "open" }, { name: "review", status: "closed" } ] }

Write an update query to change the status of the tag with name "review" to "open" using the positional $ operator.

hard

A. db.collection.updateOne({ "tags.name": "review" }, { $set: { "tags.$.name": "open" } })

B. db.collection.updateOne({ "tags.$.name": "review" }, { $set: { "tags.status": "open" } })

C. db.collection.updateOne({ "tags.name": "review" }, { $set: { "tags.status": "open" } })

D. db.collection.updateOne({ "tags.name": "review" }, { $set: { "tags.$.status": "open" } })

Solution

1. Step 1: Use correct query filter

   The filter should find the document with an array element where tags.name is "review".

2. Step 2: Use positional operator in update

   The update uses tags.$.status to update the matched tag's status to "open".

3. Final Answer:

   db.collection.updateOne({ "tags.name": "review" }, { $set: { "tags.$.status": "open" } }) -> Option D

4. Quick Check:

   Query on tags.name; update tags.$.status [OK]

Hint: Match array element in query; update with tags.$.field [OK]

Common Mistakes:

• Using $ in query filter path
• Updating wrong field in array
• Not using positional operator in update