$push operator for adding to arrays in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this $push update?

Given a document { _id: 1, fruits: ["apple", "banana"] }, what will be the fruits array after running this update?

db.collection.updateOne({ _id: 1 }, { $push: { fruits: "orange" } })

A ["apple", "banana", "orange"]

B ["orange"]

C ["apple", "banana"]

D ["apple", "banana", ["orange"]]

💡 Hint

The $push operator adds the new element to the end of the existing array.

The $push operator appends the specified value to the array field. Here, "orange" is added to the end of the existing fruits array.

📝 Syntax

intermediate

Which update query correctly uses $push to add multiple elements?

You want to add the elements "grape" and "melon" to the fruits array in one update. Which query is correct?

A db.collection.updateOne({ _id: 1 }, { $push: { fruits: { $each: ["grape", "melon"] } } })

B db.collection.updateOne({ _id: 1 }, { $push: { fruits: { $append: ["grape", "melon"] } } })

C db.collection.updateOne({ _id: 1 }, { $push: { fruits: ["grape", "melon"] } })

D db.collection.updateOne({ _id: 1 }, { $push: { fruits: { $add: ["grape", "melon"] } } })

💡 Hint

Use $each inside $push to add multiple elements.

To add multiple elements with $push, you must use the $each modifier. Options B and C use invalid operators, and A pushes the entire array as one element.

❓ optimization

advanced

How to efficiently add unique elements to an array?

You want to add "kiwi" to the fruits array only if it does not already exist. Which update query achieves this efficiently?

A db.collection.updateOne({ _id: 1 }, { $push: { fruits: "kiwi" } })

B db.collection.updateOne({ _id: 1 }, { $addToSet: { fruits: "kiwi" } })

C db.collection.updateOne({ _id: 1, fruits: { $ne: "kiwi" } }, { $push: { fruits: "kiwi" } })

D db.collection.updateOne({ _id: 1 }, { $push: { fruits: { $unique: "kiwi" } } })

💡 Hint

Use an operator designed to add unique elements to arrays.

$addToSet adds the element only if it is not already present. $push always adds the element, possibly creating duplicates.

🧠 Conceptual

advanced

What happens if you $push to a non-array field?

Consider a document { _id: 1, fruits: "apple" }. What happens if you run:

db.collection.updateOne({ _id: 1 }, { $push: { fruits: "banana" } })

A The update ignores the $push and leaves the field unchanged

B The update converts the field to an array and adds "banana"

C The update fails with an error because the field is not an array

D The field is replaced with the string "banana"

💡 Hint

Think about the data type requirements for $push.

$push requires the target field to be an array. If it is not, the update fails with an error.

🔧 Debug

expert

Why does this $push update not add the element?

You run this update:

db.collection.updateOne({ _id: 1 }, { $push: { fruits: { $each: ["pear"], $slice: 2 } } })

The fruits array was ["apple", "banana"] before. After the update, it remains unchanged. Why?

A Because $each requires $sort to work properly with $slice

B Because $slice must be negative to keep the last elements, so the update removed all elements

C Because $slice is not a valid modifier inside $push

D Because $slice with a positive number limits the array to that many elements from the start, but the update did not add new elements first

💡 Hint

Understand how $slice works with $push and $each.

$slice limits the array size after adding new elements. A positive $slice keeps only the first N elements. Since the array was already size 2, adding one element and slicing to 2 keeps only the first two elements, effectively discarding the new one.