$size (aggregation)
On this page
New in version 2.6.
Definition
$size-
Counts and returns the total number of items in an array.
$sizehas the following syntax:{ $size: <expression> }The argument for
$sizecan be any expression as long as it resolves to an array. For more information on expressions, see Expressions.
Behavior
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.
Example
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" }