$addToSet for unique array additions in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the resulting array after using $addToSet?

Consider a MongoDB document with the field tags as ["red", "blue"]. You run this update:

{ $addToSet: { tags: "green" } }

What will be the new value of tags?

A ["red", "blue", "green", "green"]

B ["red", "blue", "green"]

C ["green"]

D ["red", "blue"]

💡 Hint

Remember, $addToSet adds the value only if it is not already present.

The $addToSet operator adds the value "green" to the tags array only if it does not exist. Since "green" is not in the array, it is added, resulting in ["red", "blue", "green"].

🧠 Conceptual

intermediate

What does $addToSet do when the value already exists?

If you use $addToSet to add a value that is already in the array, what happens?

A The array remains unchanged; no duplicates are added.

B The entire array is replaced with the new value.

C The value is added again, creating duplicates.

D An error is thrown because duplicates are not allowed.

💡 Hint

Think about the meaning of 'set' in $addToSet.

$addToSet ensures that the value is only added if it does not already exist in the array. If the value is present, the array stays the same without duplicates.

📝 Syntax

advanced

Which update query correctly adds multiple unique values using $addToSet?

You want to add the values "yellow" and "purple" to the colors array, ensuring no duplicates. Which query is correct?

A { $addToSet: { colors: ["yellow", "purple"] } }

B { $push: { colors: { $each: ["yellow", "purple"] } } }

C { $addToSet: { colors: { $each: ["yellow", "purple"] } } }

D { $addToSet: { colors: { $all: ["yellow", "purple"] } } }

💡 Hint

To add multiple values uniquely, you need a special operator inside $addToSet.

The $each operator inside $addToSet allows adding multiple values uniquely. Option C correctly uses $each. Option C tries to add an array as a single element. Option C uses $push which can add duplicates. Option C uses a non-existent operator $all.

🔧 Debug

advanced

Why does this $addToSet update add the new value?

Given the document { _id: 1, items: ["apple", "banana"] }, you run this update:

{ $addToSet: { items: { name: "apple" } } }

Why does the new value get added to the array?

A Because the array already contains the exact object { name: "apple" }.

B Because $addToSet only works with strings, not objects.

C Because the update syntax is invalid and does not run.

D Because the value { name: "apple" } is an object and differs from the string "apple" in the array.

💡 Hint

Think about how MongoDB compares objects and strings in arrays.

The array contains the string "apple", but $addToSet is trying to add an object { name: "apple" }. MongoDB uses exact equality comparison (including type) for $addToSet, so the object differs from the string and is added to the array.

❓ optimization

expert

How to efficiently add unique nested objects to an array using $addToSet?

You have a collection where each document has an orders array of objects like { productId: 1, quantity: 2 }. You want to add a new order { productId: 3, quantity: 1 } only if it does not already exist (exact match). Which update query is the most efficient and correct?

A { $addToSet: { orders: { productId: 3, quantity: 1 } } }

B { $push: { orders: { productId: 3, quantity: 1 } } }

C { $addToSet: { orders: { $each: [ { productId: 3, quantity: 1 } ] } } }

D { $addToSet: { orders: { $elemMatch: { productId: 3, quantity: 1 } } } }

💡 Hint

Consider how $addToSet treats objects and the use of $each or $elemMatch.

Option A correctly uses $addToSet to add a single object if it does not exist exactly in the array. Option A uses $push which can add duplicates. Option A uses $each unnecessarily for a single object, but it still works. Option A uses $elemMatch which is not valid inside $addToSet. The most efficient and correct is option A.