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$size (aggregation)

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  • Definition
  • Compatibility
  • Syntax
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$size

Counts and returns the total number of items in an array.

You can use $size for deployments hosted in the following environments:

  • MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

  • MongoDB Enterprise: The subscription-based, self-managed version of MongoDB

  • MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

$size has the following syntax:

{ $size: <expression> }

The argument for $size can be any expression as long as it resolves to an array. For more information on expressions, see Expression Operators.

The argument for $size must resolve to an array. If the argument for $size is missing or does not resolve to an array, $size errors.

Consider an inventory collection with the following documents:

{ "_id" : 1, "item" : "ABC1", "description" : "product 1", colors: [ "blue", "black", "red" ] }
{ "_id" : 2, "item" : "ABC2", "description" : "product 2", colors: [ "purple" ] }
{ "_id" : 3, "item" : "XYZ1", "description" : "product 3", colors: [ ] }
{ "_id" : 4, "item" : "ZZZ1", "description" : "product 4 - missing colors" }
{ "_id" : 5, "item" : "ZZZ2", "description" : "product 5 - colors is string", colors: "blue,red" }

The following aggregation pipeline operation uses the $size operator to return the number of elements in the colors array:

db.inventory.aggregate([
{
$project: {
item: 1,
numberOfColors: { $cond: { if: { $isArray: "$colors" }, then: { $size: "$colors" }, else: "NA"} }
}
}
] )

The operation returns the following:

{ "_id" : 1, "item" : "ABC1", "numberOfColors" : 3 }
{ "_id" : 2, "item" : "ABC2", "numberOfColors" : 1 }
{ "_id" : 3, "item" : "XYZ1", "numberOfColors" : 0 }
{ "_id" : 4, "item" : "ZZZ1", "numberOfColors" : "NA" }
{ "_id" : 5, "item" : "ZZZ2", "numberOfColors" : "NA" }

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