MongoDB $size Operator

The $size operator in MongoDB is used to query for documents where the size of a specified array field matches a specified value. It is commonly employed within the $match stage of an aggregation pipeline to filter documents based on the size of an array. This guide will cover the syntax, examples, output, explanations, use cases, important points, and a summary of using the $size operator in MongoDB aggregation.

Syntax

{ $match: { arrayField: { $size: sizeValue } } }

• $match: Aggregation stage to filter documents.
• arrayField: The array field on which to apply the $size operator.
• $size: The operator that checks if the size of the array field is equal to the specified value.
• sizeValue: The value to compare the size of the array against.

Example

Consider a collection named orders with documents containing an items array field. We want to find orders where the number of items in the array is 3.

db.orders.aggregate([
  {
    $match: {
      "items": { $size: 3 }
    }
  }
]);

Output

The output will display documents from the orders collection where the size of the items array is 3.

[
  { "_id": ObjectId("..."), "orderNumber": "12345", "items": ["item1", "item2", "item3"] },
  { "_id": ObjectId("..."), "orderNumber": "67890", "items": ["item4", "item5", "item6"] },
  // ... other documents
]

Explanation

• The $match stage is used to filter documents based on the condition that the size of the items array field is 3.

Use

The $size operator in MongoDB is used for:

• Filtering documents based on the size of an array in the $match stage.
• Retrieving documents where an array field has a specific number of elements.

Important Points

• The $size operator is specific to array fields.
• It can be used in combination with other operators or stages to create more complex queries.

Summary

The $size operator in MongoDB provides a straightforward way to filter documents based on the size of an array field. It is useful when you need to retrieve documents where an array has a specific number of elements. Understanding how to use the $size operator is important for MongoDB developers working with aggregation pipelines to filter and analyze data.