$addToSet accumulator for unique arrays in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $addToSet accumulator do in MongoDB aggregation?

$addToSet adds unique values to an array during aggregation. It prevents duplicates, so each value appears only once.

beginner

How is $addToSet different from $push in MongoDB?

$push adds all values to an array, including duplicates. $addToSet adds only unique values, skipping duplicates.

intermediate

Write a simple example of $addToSet usage in an aggregation pipeline.

{ $group: { _id: "$category", uniqueItems: { $addToSet: "$item" } } }

This groups documents by category and collects unique items in each group.

intermediate

Can $addToSet be used to accumulate unique objects in an array?

Yes, $addToSet can accumulate unique objects. Uniqueness is based on the whole object content, not just a field.

advanced

What happens if you use $addToSet on a field that contains arrays?

$addToSet treats the entire array as one value. It adds the array only if it is unique compared to others.

What is the main purpose of $addToSet in MongoDB aggregation?

A To add unique values to an array

B To sum numeric values

C To count documents

D To sort documents

$addToSet collects unique values into an array during aggregation.

Which accumulator allows duplicates in the array?

A $avg

B $addToSet

C $sum

D $push

$push adds all values including duplicates.

If you want to group documents by a field and collect unique values of another field, which accumulator do you use?

A $addToSet

B $sum

C $max

D $min

$addToSet collects unique values during grouping.

What will $addToSet do if the same object appears multiple times?

A Add all copies

B Add only one copy

C Ignore all copies

D Throw an error

$addToSet adds only one copy of identical objects.

Can $addToSet be used to accumulate unique arrays inside an array?

A No, it flattens arrays automatically

B No, it only works with numbers

C Yes, it treats each array as a unique value

D Yes, but only with strings

$addToSet treats each array as a single value and adds it if unique.

Explain how $addToSet helps in creating arrays without duplicates in MongoDB aggregation.

Think about collecting unique items when grouping data.

Describe the difference between $addToSet and $push accumulators.

One keeps duplicates, the other does not.