$size operator for array length in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find documents where the array 'tags' has exactly 3 elements.

MongoDB

{ tags: { $[1]: 3 } }

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

AlengthOf

Blength

Ccount

D$size

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'length' or 'count' instead of '$size' causes errors.

Omitting the dollar sign before 'size'.

The $size operator matches arrays with the specified number of elements.

2fill in blank

medium

Complete the aggregation pipeline stage to add a field 'tagCount' with the length of the 'tags' array.

MongoDB

{ $addFields: { tagCount: { $[1]: "$tags" } } }

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

Alength

B$size

ClengthOf

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using operators like $length or $count which do not exist in MongoDB aggregation.

Forgetting to use quotes around the field name.

The $size operator returns the number of elements in an array field.

3fill in blank

hard

Fix the error in the query to find documents where 'items' array length is 5.

MongoDB

{ items: { $[1]: 5 } }

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

A$size

Blength

Ccount

DlengthOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using $length or $count which are invalid operators.

Missing the dollar sign before 'size'.

The correct operator to check array length in MongoDB queries is $size.

4fill in blank

hard

Fill both blanks to create a match stage that filters documents where 'comments' array length is greater than 2.

MongoDB

{ $match: { comments: { $[1]: { $[2]: 2 } } } }

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

A$size

B$gt

C$lt

D$eq

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt instead of $gt for greater than comparison.

Using $eq which checks equality, not greater than.

Use $size to check array length and $gt to compare if length is greater than 2.

5fill in blank

hard

Fill both blanks to create an aggregation stage that filters documents with 'reviews' array length equal to 4 and adds a field 'reviewCount' with that length.

MongoDB

{ $match: { reviews: { $[1]: 4 } } }, { $addFields: { reviewCount: { $[2]: "$reviews" } } }

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

A$size

Ccount

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'count' which is not a valid operator.

Using '$size' as a string in $match which expects operator without quotes.

Use $size in both $match and $addFields stages to check and assign array length.

Practice

1. What does the $size operator do in MongoDB?

easy

A. Counts the number of elements in an array

B. Calculates the sum of numbers in an array

C. Finds the largest number in an array

D. Sorts the elements of an array

Solution

1. Step 1: Understand the purpose of $size

   The $size operator is used to count how many elements are inside an array field in a MongoDB document.

2. Step 2: Compare with other options

   Other options describe different operations like sum, max, or sort, which are not what $size does.

3. Final Answer:

   Counts the number of elements in an array -> Option A

4. Quick Check:

   $size = count array elements [OK]

Hint: Remember: $size counts array items, not values [OK]

Common Mistakes:

• Confusing $size with sum or max functions
• Thinking $size sorts arrays
• Using $size on non-array fields

2. Which of the following is the correct syntax to use $size in a MongoDB aggregation pipeline to add a field itemCount that counts elements in the items array?

easy

A. { $addFields: { itemCount: { $length: "$items" } } }

B. { $match: { itemCount: { $size: "$items" } } }

C. { $project: { itemCount: { $size: "items" } } }

D. { $addFields: { itemCount: { $size: "$items" } } }

Solution

1. Step 1: Identify correct operator usage in aggregation

   The $size operator is used inside an expression to count array elements. It must be inside a stage like $addFields or $project with the array field referenced as "$items".

2. Step 2: Check syntax correctness

   { $addFields: { itemCount: { $length: "$items" } } } uses a non-existent $length. { $project: { itemCount: { $size: "items" } } } misses the $ before items. { $match: { itemCount: { $size: "$items" } } } misuses $match with $size.

3. Final Answer:

   { $addFields: { itemCount: { $size: "$items" } } } -> Option D

4. Quick Check:

   Use $size inside $addFields with "$arrayField" [OK]

Hint: Use "$arrayField" inside $size in $addFields or $project [OK]

Common Mistakes:

• Using $length instead of $size
• Forgetting the $ before array field name
• Using $size inside $match incorrectly

3. Given the collection documents:

{ "name": "Alice", "tags": ["red", "blue"] }

{ "name": "Bob", "tags": ["green"] }

{ "name": "Carol", "tags": [] }

What will be the result of this aggregation pipeline?

[{ $project: { name: 1, tagCount: { $size: "$tags" } } }]

medium

A. [{ "name": "Alice", "tagCount": 2 }, { "name": "Bob", "tagCount": 1 }, { "name": "Carol", "tagCount": 0 }]

B. [{ "name": "Alice", "tagCount": 3 }, { "name": "Bob", "tagCount": 1 }, { "name": "Carol", "tagCount": 1 }]

C. [{ "name": "Alice", "tagCount": 2 }, { "name": "Bob", "tagCount": 0 }, { "name": "Carol", "tagCount": 0 }]

D. SyntaxError

Solution

1. Step 1: Understand $size counts array elements

   For each document, $size counts how many items are in the tags array: Alice has 2, Bob has 1, Carol has 0.

2. Step 2: Apply $project to include name and tagCount

   The pipeline projects the name and adds tagCount with the counted size.

3. Final Answer:

   [{ "name": "Alice", "tagCount": 2 }, { "name": "Bob", "tagCount": 1 }, { "name": "Carol", "tagCount": 0 }] -> Option A

4. Quick Check:

   Count array lengths with $size = correct counts [OK]

Hint: Count array length per document with $size in $project [OK]

Common Mistakes:

• Assuming empty arrays count as 1
• Mixing up counts for different documents
• Expecting syntax error for correct query

4. You wrote this aggregation stage to filter documents with exactly 3 tags:

{ $match: { tags: { $size: 3 } } }

But it returns an error. What is the problem?

medium

A. The array field name is missing the $ sign

B. The $match stage requires $expr to use $size

C. The number 3 should be in quotes as "3"

D. $size cannot be used inside $match like this

Solution

1. Step 1: Understand $size usage in $match

   Directly using $size inside $match like this is invalid because $size is an aggregation expression, not a query operator.

2. Step 2: Use $expr to evaluate aggregation expressions in $match

   To filter by array length, you must use $expr with $size, like: { $match: { $expr: { $eq: [ { $size: "$tags" }, 3 ] } } }.

3. Final Answer:

   The $match stage requires $expr to use $size -> Option B

4. Quick Check:

   Use $expr for aggregation expressions in $match [OK]

Hint: Use $expr to apply $size inside $match [OK]

Common Mistakes:

• Trying to use $size directly in $match
• Forgetting $expr wrapper
• Using quotes around numbers incorrectly

5. You want to find documents where the comments array has more than 2 elements. Which aggregation pipeline stage correctly filters these documents?

hard

A. { $match: { $size: { $gt: [ "$comments", 2 ] } } }

B. { $match: { comments: { $size: { $gt: 2 } } } }

C. { $match: { $expr: { $gt: [ { $size: "$comments" }, 2 ] } } }

D. { $match: { $expr: { $size: { $gt: [ "$comments", 2 ] } } } }

Solution

1. Step 1: Use $expr to evaluate expressions in $match

   To compare array length, use $expr to allow aggregation expressions inside $match.

2. Step 2: Use $gt with $size to check array length greater than 2

   The correct syntax is { $gt: [ { $size: "$comments" }, 2 ] } inside $expr.

3. Step 3: Eliminate incorrect options

   The incorrect options either lack $expr, misuse $size placement, or have wrong syntax for $gt.

4. Final Answer:

   { $match: { $expr: { $gt: [ { $size: "$comments" }, 2 ] } } } -> Option C

5. Quick Check:

   Filter by array length with $expr and $gt [OK]

Hint: Use $expr with $gt and $size to filter by array length [OK]

Common Mistakes:

• Using $size as a query operator inside $match
• Wrong order or structure of $gt and $size
• Missing $expr wrapper