How to Use $size Operator in MongoDB for Array Length Queries

In MongoDB, the $size operator is used in queries to match documents where an array field has an exact number of elements. You use it inside a query filter like { arrayField: { $size: number } } to find documents with arrays of that specific length.

Syntax

The $size operator is used in a query filter to match documents where the specified array field has exactly the given number of elements.

Syntax parts:

• arrayField: The name of the array field in the document.
• $size: The operator that checks the array length.
• number: The exact number of elements the array must have.

json

{
  arrayField: { $size: number }
}

Example

This example shows how to find documents where the tags array has exactly 3 elements.

javascript

db.articles.insertMany([
  { title: "Article 1", tags: ["mongodb", "database", "nosql"] },
  { title: "Article 2", tags: ["mongodb"] },
  { title: "Article 3", tags: ["database", "nosql"] }
])

// Query to find documents with tags array size 3
const result = db.articles.find({ tags: { $size: 3 } }).toArray()
printjson(result)

Output

[
  {
    "_id": ObjectId("..."),
    "title": "Article 1",
    "tags": ["mongodb", "database", "nosql"]
  }
]

Common Pitfalls

Common mistakes when using $size include:

• Trying to use $size with comparison operators like $gt or $lt. $size only matches exact sizes.
• Using $size on fields that are not arrays, which will not match any documents.
• Confusing $size with $exists or $type which check for presence or type, not length.

Correct way to check for array length greater than a number is to use $expr with $gt and $size inside it.

javascript

// Incorrect: This will not work
// db.collection.find({ tags: { $size: { $gt: 2 } } })

// Correct: Use $expr to compare array length
// db.collection.find({ $expr: { $gt: [ { $size: "$tags" }, 2 ] } })

Quick Reference

Operator	Description	Example
$size	Matches arrays with exact length	{ tags: { $size: 3 } }
$expr with $size	Compare array length with operators	{ $expr: { $gt: [ { $size: "$tags" }, 2 ] } }

Key Takeaways

Use $size to match arrays with an exact number of elements in MongoDB queries.

$size cannot be combined directly with comparison operators like $gt or $lt.

To query arrays by length greater or less than a number, use $expr with $size inside.

Ensure the field you use $size on is an array to get correct results.

Use $size inside the query filter as { arrayField: { $size: number } }.