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

In MongoDB, the $size operator is used to find documents where an array field has a specific number of elements. You use it inside a query to match arrays of an exact length, like { arrayField: { $size: 3 } } to find arrays with exactly 3 items.

Syntax

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

Syntax structure:

• { arrayField: { $size: number } }

Here:

• arrayField is the name of the array field in the document.
• number is the exact length of the array you want to match.

json

{
  "arrayField": { "$size": number }
}

Example

This example shows how to find documents where the tags array has exactly 2 elements.

javascript

db.products.insertMany([
  { name: "Pen", tags: ["stationery", "writing"] },
  { name: "Notebook", tags: ["stationery"] },
  { name: "Backpack", tags: ["travel", "school", "bag"] }
])

// Query to find products with exactly 2 tags
const result = db.products.find({ tags: { $size: 2 } }).toArray()
printjson(result)

Output

[
  {
    "_id": ObjectId("..."),
    "name": "Pen",
    "tags": ["stationery", "writing"]
  }
]

Common Pitfalls

Common mistakes when using $size include:

• Trying to use $size with comparison operators like $gt or $lt. $size only matches exact array lengths.
• Using $size on fields that are not arrays will not match any documents.
• Confusing $size with $elemMatch which matches array elements by condition, not size.

Correct usage example:

{ tags: { $size: 3 } }

Incorrect usage example (won't work):

{ tags: { $size: { $gt: 2 } } }

Quick Reference

Summary tips for using $size:

• Use $size to match arrays with an exact number of elements.
• It cannot be combined with comparison operators.
• Only works on array fields.
• Use $elemMatch for matching array elements by condition.

Key Takeaways

Use $size to find documents where an array has an exact length.

$size does not support greater than or less than comparisons.

Ensure the field you query with $size is an array.

For conditional matching inside arrays, use $elemMatch instead.

The syntax is simple: { arrayField: { $size: number } }.