$addToSet accumulator for unique arrays in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to use $addToSet to add unique values to the array.

MongoDB

{ $group: { _id: "$category", items: { [1]: "$item" } } }

Drag options to blanks, or click blank then click option'

A$sum

B$push

C$addToSet

D$avg

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push instead of $addToSet, which allows duplicates.

Using $sum or $avg which are for numeric calculations, not arrays.

The $addToSet accumulator adds unique values to an array, avoiding duplicates.

2fill in blank

medium

Complete the aggregation stage to group by "type" and collect unique "tags" using $addToSet.

MongoDB

{ $group: { _id: "$type", uniqueTags: { [1]: "$tags" } } }

Drag options to blanks, or click blank then click option'

A$push

B$max

C$first

D$addToSet

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push which does not remove duplicates.

Using $first or $max which return single values, not arrays.

$addToSet collects unique values from the "tags" field into an array.

3fill in blank

hard

Fix the error in the aggregation pipeline to correctly use $addToSet for unique "colors".

MongoDB

{ $group: { _id: "$brand", colors: { [1]: "$colors" } } }

Drag options to blanks, or click blank then click option'

A$addToSet

B$avg

C$sum

D$push

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push instead of $addToSet.

Using numeric accumulators like $sum or $avg for arrays.

Only $addToSet correctly accumulates unique values in an array.

4fill in blank

hard

Fill both blanks to group by "department" and collect unique "employees" using $addToSet.

MongoDB

{ $group: { _id: "$department", [1]: { [2]: "$employee" } } }

Drag options to blanks, or click blank then click option'

Astaff

B$push

C$addToSet

Dmembers

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push instead of $addToSet.

Using field names like "members" without matching the code.

The field name can be any valid name like "staff"; $addToSet collects unique employees.

5fill in blank

hard

Fill both blanks to group by "category", collect unique "products" with $addToSet, and name the array "uniqueProducts".

MongoDB

{ $group: { _id: "$category", [1]: { [2]: "$product" } } }

Drag options to blanks, or click blank then click option'

AuniqueProducts

B$addToSet

C$push

Ditems

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push which allows duplicates.

Confusing field names or accumulator names.

The field name is "uniqueProducts", the accumulator is $addToSet, and "items" is a distractor option not used here.

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