$size operator for array length in MongoDB - Step-by-Step Execution

Concept Flow - $size operator for array length

Start with document

↓

Locate array field

↓

Apply $size operator

↓

Count elements in array

↓

Return count as result

The $size operator counts how many elements are in a specified array field of a document and returns that number.

Execution Sample

MongoDB

db.collection.aggregate([
  { $project: { name: 1, numTags: { $size: "$tags" } } }
])

This query adds a field 'numTags' showing the number of elements in the 'tags' array for each document.

Execution Table

Step	Document	Array Field	Action	Result
1	{ name: "Book A", tags: ["fiction", "bestseller"] }	tags	Apply $size	2
2	{ name: "Book B", tags: ["non-fiction"] }	tags	Apply $size	1
3	{ name: "Book C", tags: [] }	tags	Apply $size	0
4	{ name: "Book D" }	tags	Apply $size	error (field missing)

💡 All documents processed; $size returns count of array elements or error if field missing.

Variable Tracker

Variable	Start	After Doc 1	After Doc 2	After Doc 3	After Doc 4
tags	["fiction", "bestseller"]	["fiction", "bestseller"]	["non-fiction"]	[]	undefined
numTags	undefined	2	1	0	error

Key Moments - 3 Insights

What happens if the array field does not exist in a document?

The $size operator cannot count elements if the array field is missing, resulting in an error, as shown in execution_table row 4.

Does $size count nested arrays or only the top-level array elements?

$size counts only the top-level elements of the specified array field, not elements inside nested arrays.

What if the array is empty?

If the array is empty, $size returns 0, as shown in execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the $size result for the document with tags ["fiction", "bestseller"]?

A 1

B 2

C 0

D error

💡 Hint

Check execution_table row 1 under Result column.

At which step does $size return 0?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table rows and find where the array is empty.

If a document has no 'tags' field, what will $size return according to the execution_table?

A error

B 0

C Number of elements in another field

D 1

💡 Hint

See execution_table row 4 Result column.

Concept Snapshot

$size operator counts elements in an array field.
Syntax: { $size: "<arrayField>" }
Returns integer count of array elements.
Returns error if field missing or not an array.
Used in aggregation pipelines to project array length.

Full Transcript

The $size operator in MongoDB counts how many items are inside an array field of a document. When you run an aggregation query with $size, it looks at each document, finds the specified array, counts its elements, and returns that number. If the array is empty, it returns zero. If the array field is missing, it results in an error. This operator is useful to quickly know how many items are in an array without manually counting them.

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