$size operator for array length in MongoDB

Introduction

The $size operator helps you find out how many items are in an array inside your data. It tells you the length of the array.

You want to count how many hobbies a person has in their profile.

You need to find products that have exactly 3 tags.

You want to filter orders that contain more than 5 items.

You want to check if a student's list of completed courses is empty or not.

Syntax

MongoDB

{ $size: <array> }

The must be a field that contains an array in your document.

You can use $size inside aggregation pipelines or queries to filter documents.

Examples

Counts the number of elements in the 'hobbies' array.

MongoDB

{ $size: "$hobbies" }

Returns the length of the 'tags' array.

MongoDB

{ $size: "$tags" }

Finds how many items are in the 'items' array inside 'orders'.

MongoDB

{ $size: "$orders.items" }

Sample Program

This program inserts three users with different numbers of hobbies. Then it uses $size to count hobbies for each user and shows the result.

MongoDB

db.users.insertMany([
  { name: "Alice", hobbies: ["reading", "swimming", "coding"] },
  { name: "Bob", hobbies: ["gaming"] },
  { name: "Charlie", hobbies: [] }
])

// Find users and show how many hobbies they have

const cursor = db.users.aggregate([
  {
    $project: {
      name: 1,
      hobbiesCount: { $size: "$hobbies" }
    }
  }
])

cursor.forEach(doc => printjson(doc))

Important Notes

The $size operator runs in O(1) time because MongoDB stores array lengths internally.

If the field is not an array or is missing, $size throws an error in aggregation pipelines; in queries, it matches documents where the field is an array of the specified size.

Use $size when you want exact array length; for conditions like 'more than' or 'less than', combine with $expr and comparison operators.

Summary

$size tells you how many elements are in an array.

Use it to count items inside arrays in your documents.

Works well inside aggregation pipelines to create new fields or filter data.

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