Array update with $[identifier] filtered in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Update specific array elements using $[identifier]

Given the following document in a collection products:

{
  _id: 1,
  name: "T-Shirt",
  sizes: [
    { size: "S", stock: 10 },
    { size: "M", stock: 5 },
    { size: "L", stock: 0 }
  ]
}

What will be the resulting document after running this update?

db.products.updateOne(
  { _id: 1 },
  { $inc: { "sizes.$[elem].stock": 3 } },
  { arrayFilters: [ { "elem.stock": { $lt: 5 } } ] }
)

A { _id: 1, name: "T-Shirt", sizes: [ { size: "S", stock: 13 }, { size: "M", stock: 5 }, { size: "L", stock: 3 } ] }

B { _id: 1, name: "T-Shirt", sizes: [ { size: "S", stock: 13 }, { size: "M", stock: 8 }, { size: "L", stock: 3 } ] }

C { _id: 1, name: "T-Shirt", sizes: [ { size: "S", stock: 10 }, { size: "M", stock: 5 }, { size: "L", stock: 3 } ] }

D { _id: 1, name: "T-Shirt", sizes: [ { size: "S", stock: 10 }, { size: "M", stock: 8 }, { size: "L", stock: 3 } ] }

💡 Hint

Look at which array elements have stock less than 5 before the update.

The arrayFilters condition { "elem.stock": { $lt: 5 } } matches sizes with stock less than 5. Initially, only sizes "M" (5) and "L" (0) are checked. Since 5 is not less than 5, only "L" matches. So only "L" stock is incremented by 3 from 0 to 3.

📝 Syntax

intermediate

Identify the syntax error in array update with $[identifier]

Which of the following update commands will cause a syntax error when trying to update array elements with $[identifier]?

db.orders.updateOne(
  { _id: 101 },
  { $set: { "items.$[item].status": "shipped" } },
  { arrayFilters: [ { "item.qty": { $gte: 2 } } ] }
)

A db.orders.updateOne({ _id: 101 }, { $set: { "items.$[item].status": "shipped" } }, { arrayFilters: [ { "item.qty": { $gte 2 } } ] })

B db.orders.updateOne({ _id: 101 }, { $set: { "items.$[item].status": "shipped" } }, { arrayFilters: [ { "item.qty": { $gte: 2 } } ] })

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D b.orders.updateOne({ _id: 101 }, { $set: { "items.$[item].status": "shipped" } }, { arrayFilters: [ { "item.qty": { $gte: 2 } } ] })

💡 Hint

Check the syntax of the $gte operator in the array filter.

Option A is missing a colon between $gte and 2, causing a syntax error. The correct syntax is { $gte: 2 }.

❓ optimization

advanced

Optimize update to increment multiple array elements conditionally

You want to increment the score by 1 for all players in the players array whose level is greater than 10.

Which update command is the most efficient and correct?

A db.games.updateOne({ _id: 5 }, { $inc: { "players.$[p].score": 1 } })

B db.games.updateMany({}, { $inc: { "players.$[p].score": 1 } }, { arrayFilters: [ { "p.level": { $gt: 10 } } ] })

C db.games.updateOne({ _id: 5 }, { $inc: { "players.$.score": 1 } }, { arrayFilters: [ { "p.level": { $gt: 10 } } ] })

D db.games.updateOne({ _id: 5 }, { $inc: { "players.$[p].score": 1 } }, { arrayFilters: [ { "p.level": { $gt: 10 } } ] })

💡 Hint

Consider the filter on the document and the use of arrayFilters to target multiple array elements.

Option D correctly updates one document with _id 5 and increments score for all players with level > 10 using arrayFilters. Option D updates many documents unnecessarily. Option D uses positional operator $ without arrayFilters, which updates only the first matching element. Option D lacks arrayFilters, so it cannot filter array elements.

🔧 Debug

advanced

Why does this array update not change any elements?

Given this document:

{ _id: 10, tasks: [ { name: "task1", done: false }, { name: "task2", done: false } ] }

And this update command:

db.projects.updateOne(
  { _id: 10 },
  { $set: { "tasks.$[t].done": true } },
  { arrayFilters: [ { "t.done": true } ] }
)

Why does this update not change any elements?

A Because the positional operator $ cannot be used with arrayFilters.

B Because the array filter matches elements where done is true, but all are false initially.

C Because the update query is missing the filter on _id: 10.

D Because the update operator $set cannot be used on array elements.

💡 Hint

Check the condition inside arrayFilters and the initial values.

The array filter { "t.done": true } matches only elements where done is true. Since all tasks have done false, no elements match, so no update occurs.

🧠 Conceptual

expert

Understanding multiple arrayFilters with different identifiers

Consider a document:

{
  _id: 20,
  orders: [
    { id: 1, items: [ { product: "A", qty: 2 }, { product: "B", qty: 1 } ] },
    { id: 2, items: [ { product: "A", qty: 1 }, { product: "C", qty: 5 } ] }
  ]
}

You want to increment the quantity by 1 for product "A" only in order with id 2.

Which update command achieves this?

A

db.sales.updateOne(
  { _id: 20 },
  { $inc: { "orders.$[order].items.$[item].qty": 1 } },
  { arrayFilters: [ { "order.id": 2 }, { "item.product": "A" } ] }
)

B

db.sales.updateOne(
  { _id: 20 },
  { $inc: { "orders.$[item].items.$[order].qty": 1 } },
  { arrayFilters: [ { "order.id": 2 }, { "item.product": "A" } ] }
)

C

db.sales.updateOne(
  { _id: 20 },
  { $inc: { "orders.$[order].items.$[item].qty": 1 } },
  { arrayFilters: [ { "item.id": 2 }, { "order.product": "A" } ] }
)

D

db.sales.updateOne(
  { _id: 20 },
  { $inc: { "orders.$.items.$[item].qty": 1 } },
  { arrayFilters: [ { "item.product": "A" } ] }
)

💡 Hint

Use two different identifiers in arrayFilters to target nested arrays.

Option A correctly uses two identifiers: order to filter orders with id 2, and item to filter items with product "A". The update increments qty by 1 only for matching nested elements.

Other options have swapped identifiers or wrong filter fields.