$each modifier with $push in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output after using $push with $each?

Consider a MongoDB collection students with a document:

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

We run this update:

db.students.updateOne({ _id: 1 }, { $push: { scores: { $each: [95, 100] } } })

What will be the scores array after this update?

A [85, 90]

B [95, 100]

C [85, 90, [95, 100]]

D [85, 90, 95, 100]

💡 Hint

The $each modifier adds multiple values individually to the array.

The $push operator adds elements to an array. When combined with $each, it adds each element from the given array separately. So the original array [85, 90] gets 95 and 100 appended individually, resulting in [85, 90, 95, 100].

🧠 Conceptual

intermediate

Why use $each with $push instead of pushing an array directly?

In MongoDB, what is the main reason to use $each with $push when adding multiple elements to an array?

A To sort the array after pushing

B To replace the entire array with the new array

C To add each element individually instead of nesting the array inside the array

D To remove duplicate elements automatically

💡 Hint

Think about what happens if you push an array without $each.

Using $push with $each adds each element of the array separately. Without $each, pushing an array adds the whole array as a single nested element, which is usually not desired.

📝 Syntax

advanced

Identify the correct syntax for using $push with $each and $slice

Which of the following MongoDB update statements correctly uses $push with $each and $slice to add elements and keep only the last 3 elements in the array?

A db.collection.updateOne({}, { $push: { items: { $each: [1,2,3], $slice: -3 } } })

B db.collection.updateOne({}, { $push: { items: { $slice: -3, $each: [1,2,3] } } })

C db.collection.updateOne({}, { $push: { items: { $each: [1,2,3], $slice: 3 } } })

D db.collection.updateOne({}, { $push: { items: { $each: [1,2,3], $sort: 1, $slice: 3 } } })

💡 Hint

The order of modifiers inside $push matters and $slice uses positive or negative integers.

The correct syntax places $each first, then $slice. Using $slice: -3 keeps the last 3 elements. Option A uses $slice: -3 which keeps the last 3 elements after pushing.

🔧 Debug

advanced

Why does this $push with $each update fail?

Given this update command:

db.products.updateOne({ _id: 10 }, { $push: { tags: { $each: "new", $position: 0 } } })

Why does this update fail?

A Because $each requires an array, but "new" is a string

B Because $position cannot be used with $each

C Because $push cannot add elements at position 0

D Because the document with _id 10 does not exist

💡 Hint

Check the data type expected by $each.

$each expects an array of elements to add. Passing a string "new" causes a type error because a string is not an array.

❓ optimization

expert

Optimizing multiple $push operations with $each

You want to add multiple tags to a document's tags array in MongoDB. Which update operation is the most efficient and atomic way to add "red", "blue", and "green" tags at once?

A Replace the entire tags array with ["red", "blue", "green"] using $set

B Use { $push: { tags: { $each: ["red", "blue", "green"] } } } in one update

C Use $addToSet three times in one update to add each tag

D Run three separate updates each with $push: { tags: "red" }, then "blue", then "green"

💡 Hint

Think about atomicity and minimizing database operations.

Using $push with $each adds multiple elements in one atomic update, which is efficient and avoids multiple round-trips. Running multiple updates or replacing the array is less efficient or destructive.