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- $size (aggregation)
$size (aggregation)¶
2.6 新版功能.
Definition¶
- $size¶
Counts and returns the total the number of items in an array.
$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 Expressions.
Example¶
Consider a 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: [ ] }
The following aggregation pipeline operation use the $size to return the number of elements in the colors array:
db.inventory.aggregate(
[
{
$project: {
item: 1,
numberOfColors: { $size: "$colors" }
}
}
]
)
The operation returns the following:
{ "_id" : 1, "item" : "ABC1", "numberOfColors" : 3 }
{ "_id" : 2, "item" : "ABC2", "numberOfColors" : 1 }
{ "_id" : 3, "item" : "XYZ1", "numberOfColors" : 0 }