Array update with positional $ operator in MongoDB

Introduction

The positional $ operator helps you update a specific item inside an array in a document without changing the whole array.

You want to change the price of a specific product in a shopping cart array.

You need to update the status of a particular task inside a list of tasks.

You want to fix a typo in one comment inside an array of comments.

You want to mark one item as completed in a list of to-do items.

Syntax

MongoDB

db.collection.updateOne(
  { "arrayField.identifier": value },
  { $set: { "arrayField.$.fieldToUpdate": newValue } }
)

The query part finds the document with the array item matching the condition.

The $ in arrayField.$ means the first matched array element.

Examples

Updates the quantity of the first item with productId 123 in the items array.

MongoDB

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

Marks the step with stepId 10 as done in the steps array.

MongoDB

db.tasks.updateOne(
  { "steps.stepId": 10 },
  { $set: { "steps.$.status": "done" } }
)

Changes the email address in the contacts array where type is email.

MongoDB

db.users.updateOne(
  { "contacts.type": "email" },
  { $set: { "contacts.$.value": "[e-mail]" } }
)

Sample Program

This example inserts a document with an array of items. Then it updates the quantity of the item with sku 'def' to 50 using the positional $ operator.

MongoDB

db.inventory.insertOne({
  _id: 1,
  items: [
    { sku: "abc", qty: 10 },
    { sku: "def", qty: 20 },
    { sku: "xyz", qty: 30 }
  ]
})

print('Before update:')
printjson(db.inventory.findOne({_id: 1}))

// Update qty of item with sku 'def' to 50

const updateResult = db.inventory.updateOne(
  { "items.sku": "def" },
  { $set: { "items.$.qty": 50 } }
)

print('Update result:')
printjson(updateResult)

print('After update:')
printjson(db.inventory.findOne({_id: 1}))

Important Notes

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

If no array element matches, no update happens.

Use updateOne to update a single document, or updateMany to update multiple documents.

Summary

The positional $ operator lets you update one matching item inside an array.

You find the document and array element by a query, then use $set with arrayField.$.

This is useful to change just one part of an array without replacing the whole array.

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