$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.

Practice

1. What does the $addToSet accumulator do in a MongoDB aggregation pipeline?

easy

A. Counts the total number of documents in a group

B. Adds all values including duplicates into an array

C. Sorts the array elements in ascending order

D. Collects unique values into an array, avoiding duplicates

Solution

1. Step 1: Understand $addToSet purpose

   $addToSet collects values but only keeps unique ones, no duplicates.

2. Step 2: Compare with other accumulators

   Unlike $push which adds all values, $addToSet ensures uniqueness.

3. Final Answer:

   Collects unique values into an array, avoiding duplicates -> Option D

4. Quick Check:

   $addToSet = unique array [OK]

Hint: Remember: $addToSet means add unique only [OK]

Common Mistakes:

• Confusing $addToSet with $push
• Thinking it sorts the array
• Assuming it counts documents

2. Which of the following is the correct syntax to use $addToSet in a MongoDB aggregation $group stage?

easy

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

B. { $group: { _id: "$category", uniqueItems: { $push: "$item" } } }

C. { $group: { _id: "$category", uniqueItems: { $addToSet: ["$item"] } } }

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

Solution

1. Step 1: Identify correct $addToSet usage

   The correct syntax uses $addToSet with a field path as a string, like "$item".

2. Step 2: Check other options for errors

   { $group: { _id: "$category", uniqueItems: { $push: "$item" } } } uses $push, not $addToSet. { $group: { _id: "$category", uniqueItems: { $addToSet: ["$item"] } } } wrongly wraps field in array. { $group: { _id: "$category", uniqueItems: { $addToSet: { $item } } } } uses braces incorrectly.

3. Final Answer:

   { $group: { _id: "$category", uniqueItems: { $addToSet: "$item" } } } -> Option A

4. Quick Check:

   Correct syntax = { $group: { _id: "$category", uniqueItems: { $addToSet: "$item" } } } [OK]

Hint: Use field path as string inside $addToSet [OK]

Common Mistakes:

• Wrapping field in array inside $addToSet
• Using $push instead of $addToSet
• Using braces {} incorrectly around field

3. Given the collection documents:

{ "category": "fruit", "item": "apple" }

{ "category": "fruit", "item": "banana" }

{ "category": "fruit", "item": "apple" }

What will be the result of this aggregation?

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

medium

A. { "_id": "fruit", "uniqueItems": ["apple", "banana"] }

B. { "_id": "fruit", "uniqueItems": ["apple", "apple", "banana"] }

C. { "_id": "fruit", "uniqueItems": ["banana"] }

D. { "_id": "fruit", "uniqueItems": [] }

Solution

1. Step 1: Group by category and collect unique items

   All documents have category "fruit". $addToSet collects unique "item" values.

2. Step 2: Identify unique items

   Items are "apple", "banana", and another "apple" (duplicate). Unique set is ["apple", "banana"].

3. Final Answer:

   { "_id": "fruit", "uniqueItems": ["apple", "banana"] } -> Option A

4. Quick Check:

   Unique items = ["apple", "banana"] [OK]

Hint: Duplicates removed, only unique values remain [OK]

Common Mistakes:

• Expecting duplicates in result array
• Confusing $addToSet with $push
• Ignoring grouping key

4. You wrote this aggregation but it throws an error:

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

What is the error and how to fix it?

medium

A. Wrong accumulator; use $push instead of $addToSet

B. Missing quotes around field path; fix by using "$item"

C. Missing $unwind stage before $group

D. Cannot use $addToSet inside $group

Solution

1. Step 1: Identify syntax error in field reference

   Field paths must be strings with "$" prefix inside accumulators. Here, item is missing quotes and $.

2. Step 2: Correct the syntax

   Change item to "$item" to fix the error.

3. Final Answer:

   Missing quotes around field path; fix by using "$item" -> Option B

4. Quick Check:

   Field path needs "$" and quotes [OK]

Hint: Always quote field paths with $ inside accumulators [OK]

Common Mistakes:

• Forgetting $ before field name
• Not quoting field path as string
• Assuming $unwind is always required

5. You have documents with a field tags as arrays, some with duplicates:

{ "_id": 1, "tags": ["red", "blue", "red"] }

{ "_id": 2, "tags": ["blue", "green"] }

Which aggregation pipeline will produce a single array of all unique tags across documents?

hard

A. [{ $project: { uniqueTags: { $addToSet: "$tags" } } }]

B. [{ $group: { _id: null, allTags: { $addToSet: "$tags" } } }]

C. [{ $unwind: "$tags" }, { $group: { _id: null, allTags: { $addToSet: "$tags" } } }]

D. [{ $unwind: "$tags" }, { $group: { _id: "$tags", count: { $sum: 1 } } }]

Solution

1. Step 1: Use $unwind to flatten tags arrays

   $unwind breaks each array element into separate documents, allowing unique collection.

2. Step 2: Group all tags into a unique set

   Grouping by null collects all tags into one array with $addToSet ensuring uniqueness.

3. Final Answer:

   [{ $unwind: "$tags" }, { $group: { _id: null, allTags: { $addToSet: "$tags" } } }] -> Option C

4. Quick Check:

   Unwind then addToSet = unique combined array [OK]

Hint: Unwind arrays before addToSet to get unique elements [OK]

Common Mistakes:

• Trying to addToSet whole arrays without unwind
• Using $project instead of $group for aggregation
• Grouping by tags instead of null for combined result