$addToSet for unique array additions in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a unique value to the 'tags' array using $addToSet.

MongoDB

db.collection.updateOne({ _id: 1 }, { $[1]: { tags: "mongodb" } })

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

AaddToSet

Bpush

Cset

DaddUnique

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push instead of $addToSet, which can add duplicates.

Using $set which replaces the whole array.

The $addToSet operator adds a value to an array only if it does not already exist, ensuring uniqueness.

2fill in blank

medium

Complete the code to add multiple unique values to the 'tags' array using $addToSet with $each.

MongoDB

db.collection.updateOne({ _id: 2 }, { $addToSet: { tags: { $[1]: ["nodejs", "database"] } } })

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

Aall

Bmultiple

Cmany

Deach

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push with $each but forgetting $addToSet for uniqueness.

Using incorrect modifier names like $all or $many.

The $each modifier allows adding multiple values uniquely with $addToSet.

3fill in blank

hard

Fix the error in the update command to correctly add a unique value to the 'categories' array.

MongoDB

db.collection.updateOne({ _id: 3 }, { $[1]: { categories: "tech" } })

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

Apush

BaddToSet

Cset

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push which can add duplicates.

Using $set which replaces the whole array.

To add a unique value to an array, use $addToSet. Using $push adds duplicates.

4fill in blank

hard

Fill both blanks to add multiple unique tags to the 'labels' array.

MongoDB

db.collection.updateOne({ _id: 4 }, { $[1]: { labels: { $[2]: ["urgent", "review"] } } })

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

AaddToSet

Bpush

Ceach

Dset

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push instead of $addToSet.

Forgetting to use $each when adding multiple values.

Use $addToSet with $each to add multiple unique values to an array.

5fill in blank

hard

Fill all three blanks to add a unique uppercase tag to the 'tags' array using an aggregation pipeline update.

MongoDB

db.collection.updateOne({ _id: 5 }, [ { $[1]: { [2]: { $toUpper: "[3]" } } } ])

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

AconcatArrays

Burgent

CaddToSet

Dtags

Attempts:

3 left

💡 Hint

Common Mistakes

Using $concatArrays which rebuilds the array but allows duplicates.

Using $set which replaces the array.

Wrong field name or forgetting to uppercase.

In an aggregation pipeline update stage, use $addToSet (BLANK_1) on the tags field (BLANK_2) with the uppercase expression on urgent (BLANK_3) to add a unique uppercase tag.

Practice

1. What is the main purpose of the $addToSet operator in MongoDB?

easy

A. To sort the elements of an array

B. To remove duplicate values from an array

C. To replace an entire array with a new value

D. To add a value to an array only if it does not already exist

Solution

1. Step 1: Understand the purpose of $addToSet

   $addToSet adds a value to an array only if that value is not already present, ensuring uniqueness.

2. Step 2: Compare with other array operations

   Unlike operators that remove duplicates or replace arrays, $addToSet only adds unique values without duplicates.

3. Final Answer:

   To add a value to an array only if it does not already exist -> Option D

4. Quick Check:

   $addToSet ensures unique additions [OK]

Hint: Remember: $addToSet adds only new unique values [OK]

Common Mistakes:

• Confusing $addToSet with $push which adds duplicates
• Thinking $addToSet removes duplicates from existing array
• Assuming $addToSet sorts the array

2. Which of the following is the correct syntax to add multiple unique values to an array field tags using $addToSet?

easy

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

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

C. { $addToSet: { tags: { $push: ["mongodb", "database"] } } }

D. { $push: { tags: { $each: ["mongodb", "database"] } } }

Solution

1. Step 1: Recall syntax for adding multiple unique values

   To add multiple unique values with $addToSet, use $each inside it.

2. Step 2: Analyze options

   { $addToSet: { tags: { $each: ["mongodb", "database"] } } } correctly uses $addToSet with $each to add multiple unique values. { $addToSet: { tags: ["mongodb", "database"] } } lacks $each, so it tries to add an array as a single element. Using $push inside or instead of $addToSet is incorrect as it doesn't ensure uniqueness.

3. Final Answer:

   { $addToSet: { tags: { $each: ["mongodb", "database"] } } } -> Option A

4. Quick Check:

   Use $addToSet with $each for multiple unique additions [OK]

Hint: Use $each inside $addToSet to add multiple unique items [OK]

Common Mistakes:

• Omitting $each when adding multiple values
• Using $push instead of $addToSet for uniqueness
• Adding an array as a single element instead of multiple

3. Given the document { _id: 1, colors: ["red", "blue"] }, what will be the colors array after running this update?

{ $addToSet: { colors: "blue" } }

medium

A. ["red", "blue", "blue"]

B. ["red", "blue"]

C. ["blue"]

D. ["red"]

Solution

1. Step 1: Understand $addToSet behavior with existing value

   The value "blue" already exists in the array, so $addToSet will not add it again.

2. Step 2: Determine the resulting array

   Since "blue" is present, the array remains unchanged as ["red", "blue"].

3. Final Answer:

   ["red", "blue"] -> Option B

4. Quick Check:

   Existing values are not duplicated by $addToSet [OK]

Hint: If value exists, $addToSet does not add it again [OK]

Common Mistakes:

• Assuming $addToSet always appends the value
• Confusing $addToSet with $push which allows duplicates
• Thinking the array will be replaced

4. You want to add multiple unique tags to a document's tags array, but your update query:

{ $addToSet: { tags: ["nodejs", "mongodb"] } }

adds the entire array as one element instead of separate tags. What is the fix?

medium

A. Use $addToSet: { tags: { $each: ["nodejs", "mongodb"] } }

B. Use $push: { tags: ["nodejs", "mongodb"] }

C. Use $addToSet: { tags: "nodejs", "mongodb" }

D. Use $addToSet: { tags: { $push: ["nodejs", "mongodb"] } }

Solution

1. Step 1: Identify the problem with adding an array directly

   Adding an array directly with $addToSet adds it as a single element, not multiple elements.

2. Step 2: Use $each to add multiple unique elements

   Wrapping the array with $each inside $addToSet adds each element uniquely.

3. Final Answer:

   Use $addToSet: { tags: { $each: ["nodejs", "mongodb"] } } -> Option A

4. Quick Check:

   Use $each inside $addToSet for multiple unique additions [OK]

Hint: Wrap multiple values with $each inside $addToSet [OK]

Common Mistakes:

• Adding array directly without $each
• Using $push instead of $addToSet for uniqueness
• Incorrect syntax with multiple keys inside $addToSet

5. You have a MongoDB document:

{ _id: 1, items: ["apple", "banana"] }

You want to add the items ["banana", "cherry", "apple", "date"] uniquely to the items array. Which update query will correctly add only the new unique items?

hard

A. { $addToSet: { items: ["banana", "cherry", "apple", "date"] } }

B. { $set: { items: ["banana", "cherry", "apple", "date"] } }

C. { $addToSet: { items: { $each: ["banana", "cherry", "apple", "date"] } } }

D. { $push: { items: { $each: ["banana", "cherry", "apple", "date"] } } }

Solution

1. Step 1: Understand the goal to add only new unique items

   We want to add only items not already in the array, so duplicates like "banana" and "apple" should not be added again.

2. Step 2: Choose the correct operator and syntax

   Using $addToSet with $each adds multiple unique items. { $addToSet: { items: { $each: ["banana", "cherry", "apple", "date"] } } } correctly uses this syntax. { $addToSet: { items: ["banana", "cherry", "apple", "date"] } } adds the entire array as one element. { $push: { items: { $each: ["banana", "cherry", "apple", "date"] } } } uses $push which allows duplicates. { $set: { items: ["banana", "cherry", "apple", "date"] } } replaces the entire array, losing existing items.

3. Final Answer:

   { $addToSet: { items: { $each: ["banana", "cherry", "apple", "date"] } } } -> Option C

4. Quick Check:

   $addToSet with $each adds only new unique items [OK]

Hint: Use $addToSet with $each to add multiple unique items without duplicates [OK]

Common Mistakes:

• Adding array directly without $each causing one element addition
• Using $push which allows duplicates
• Replacing array with $set losing existing data