$addToSet for unique array additions in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $addToSet operator do in MongoDB?

$addToSet adds a value to an array only if the value is not already present, ensuring unique elements.

beginner

How is $addToSet different from $push in MongoDB?

$push adds values to an array regardless of duplicates, while $addToSet adds only if the value is not already in the array.

beginner

Write a MongoDB update query snippet to add the value "apple" uniquely to the fruits array.

{ $addToSet: { fruits: "apple" } }

intermediate

Can $addToSet add multiple unique values at once? If yes, how?

Yes, by using $each inside $addToSet to add multiple unique values.

{ $addToSet: { fruits: { $each: ["apple", "banana"] } } }

intermediate

What happens if you use $addToSet on a field that is not an array?

If the field does not exist, MongoDB creates an array with the value. If the field exists and holds a non-array value, the update operation fails and MongoDB returns an error.

What does $addToSet ensure when adding elements to an array?

A No duplicate elements are added

B Elements are always added at the start

C Elements are removed if they exist

D Array is replaced completely

$addToSet adds elements only if they are not already present, preventing duplicates.

Which operator would add duplicates to an array in MongoDB?

A $push

B $addToSet

C $set

D $unset

$push adds elements to an array regardless of duplicates.

How do you add multiple unique values to an array using $addToSet?

A Use <code>$push</code> with an array

B Use multiple <code>$addToSet</code> operators

C Use <code>$each</code> inside <code>$addToSet</code>

D Use <code>$set</code> with an array

$each allows adding multiple unique values at once inside $addToSet.

If the target field does not exist, what does $addToSet do?

A Throws an error

B Creates the field as a string

C Ignores the update

D Creates the field as an array with the value

$addToSet creates the field as an array containing the value if it does not exist.

Which of these is a valid use of $addToSet?

A { $addToSet: { tags: "mongodb" } }

B { $addToSet: { tags: { $each: ["mongodb", "database"] } } }

C { $addToSet: { tags: { $each: "mongodb" } } }

D { $addToSet: { tags: ["mongodb", "database"] } }

Option B correctly uses $each with an array inside $addToSet to add multiple unique values.

Explain how $addToSet helps maintain unique elements in a MongoDB array.

Think about how sets work in real life, like a collection of unique stickers.

Describe the difference between $addToSet and $push when updating arrays in MongoDB.

Imagine adding items to a shopping list with or without checking if they are already there.